That's an interesting challenge. I'll see if I can write a mini review. I should mention that I have lived with the HD6 since late last year. I have only heard the KEF LS50 in store once. I have just put a deposit on a much more expensive system, and after spending some hours auditioning my choices, I ran out of time to inquire about getting some AV furniture, ethernet cable etc, so I came back to the store the next day, and the staff were quite relaxed about letting me audition the KEF LS50 actives knowing that I wasn't planning to purchase. I could just chat with them and play them for a while without any pressure to buy.
Audioengine HD6 vs KEF LS50 Wireless Active Speaker Comparison
Very similar. If these are going into a small listening space on speaker stands, then neither will likely present an issue. The KEFs are 5.8cm deeper though, which can be an issue if these are sitting on your computer desk. Audioengine offer desktop speaker stands that are highly recommended.
HD6: (H x W x D) 300 x 185 x 250mm
KEF: (H x W x D) 300 x 200 x 308mm
The KEFs with their range of stylish finishes beat the HD6 any day. That said, the wood veneer finishes offered by Audioengine are more than decent. Mine are in Cherry, which is nowhere near as ruddy as the shots in the Audioengine's promo photos.
3. Technical Specs
The Audioengine allows you to connect wirelessly over Bluetooth, but these aren't true full active speakers. The left speaker is the active unit and this drives the right speaker passively. You need a length of speaker cable connecting them. I have upgraded the right speaker cable after finding an eBay seller willing to sell a single length of terminated 2m speaker cable. The advantage of this is that I found you get better sound when I connected from my iMac 3.5mm mini line out to the left speaker. This meant I was now using the iMac's native DAC, but it actually sounded a tiny bit better anyway. Things started to really sound astonishingly high-end only when I got an Audioquest Dragonfly Red (a Computer Audiophile recommended DAC—justly so), then connected the 3.5mm output from my iMac into the HD6, bypassing the internal DAC of both the iMac and the HD6.
From the slightly obscure technical documentation, I get the impression that if you connect the HD6 wireless via Bluetooth you are inputting a less than 24-bit signal, which the HD6 then "upsamples" (sic) to 24 bit. To make full use of the HD6's internal DAC's ability to handle 24/192 digital inputs, you have to use the Toslink input. My iMac has no Toslink out so I can't use this. If you happen to have a computer with a Toslink line out then this might be a useful feature. The best way to overcome this limitation is to buy an external DAC as your budget allows. My suggestions are the Audioquest Dragonfly Red or perhaps one of the iFi-Audio DAC—I first got the Dragonfly before upgrading to a Schiit DAC myself and connect via RCA cable from the DAC to the active left speaker unit of the HD6.
The KEFs are true wireless speakers, with both speakers being active units and each unit connects via a power plug into the mains. This is good if you want to connect wirelessly and minimise cable clutter, freeing you up to place the speakers anywhere you want. The product info sheet suggests that wireless inputs are at full 24/192 resolution. KEF states that the LS50 can accept up to 192kHz via USB Type B and up to 96kHz via TOSLINK Optical. If you want to connect the LS50 to your computer via USB A to B, you connect from computer > USB cable > right speaker > supplied ethernet cable out from right speaker > left speaker. If you don't want to upgrade to an external DAC later the KEF is the superior option to minimise component clutter provided you are connecting wirelessly. If you do connect from an external DAC to your LS50s, you connect both of these going DAC > RCA (or Toslink) cable (left and right) into right speaker > ethernet cable out from right speaker > left speaker. That means just as much cable clutter as with the HD6s—more if you consider that the LS50s need two power cables.
Read the connectivity details carefully to see if it suits your setup.
HD6: Analogue dual-class A/B monolithic 75W per channel power output.
LS50: Total of 230W per channel bi-amp dual mono configuration. 200W Class D amplifier for the 130mm magnesium/aluminium alloy bass/mid driver and a 30W Class A/B amp for the less power-hungry 25mm vented aluminium dome tweeter sitting in its centre.
The amplifier specs of the LS50 far surpass that of the HD6: you get what you pay for here. The HD6 costs AUD $1K, the LS50 Wireless costs $3.4K. The end result is that you can upgrade your signal source no end and the LS50s will always have far superior soundstaging.
4. Sound Quality
If you connect wirelessly using the internal DACs, then the LS50 will almost certainly run rings around the HD6. Straight out of the box, the LS50s are a more complete package. Much ink has been spilt about the technology behind the speaker drivers used on the LS50. The tweeter sounds wonderful and the midrange is stunning. What bass you get from the LS50s is of exceptional clarity and quality. For the HD6 to sound their best, you will need to spend another $510-570 AUD on the S8 subwoofer after upgrading the DAC (source always comes first). If you want to run the system without a subwoofer to reduce clutter get the LS50s.
That said, the HD6 is no slouch either. The silk dome tweeters are made by Audioengine themselves, and they are very refined indeed. They sound sweet and open. In fact, the silk dome tweeters on the HD6 possibly sound a touch sweeter than the metal dome tweeters on the KEFs (whether you prefer the sound of silk dome vs metal dome tweeters is a matter of personal preference). The Audioengine tweeters don't sound excessively peaky or rolled off in any way. Here are John Atkinson's measurements of the A2+ speakers:
JA concludes: "A heck of a speaker at a heck of a price!" I couldn't agree more. Usually, if a cheaper speaker from the same maker measures this ridiculously well, you can be reassured that their top of the range model in the HD6 will measure just as well and probably better. Note in particular the even frequency response curve and the clean waterfall plot. The soundstaging is remarkable, though not quite as spectacular as the LS50s. The voicing of the midrange can more than stand comparison with the LS50.
The HD6 only really comes to life to be able to stand comparison against the LS50 Wireless if you are willing to upgrade to a better DAC, plus decent cabling, and then you add a subwoofer to sit underneath your computer desk. However, the fact that with a bit of tweaking the HD6s can more than stand comparison against the LS50 Wireless is in itself remarkable.
If you want to buy and then forget the rest, then the KEF LS50 is the no-brainer choice for running a wireless setup. It's a gorgeous piece of industrial design and it is astonishing that something so small can give even the most expensive high-end systems a run for your money. You don't have to worry about the synergy between component parts, the KEF engineers have done all that work for you. The result will outperform most separates at the same price point. For many, the reduction in cable clutter will make the KEFs the natural choice. The store had Dynaudio Evidence speakers driven by MacIntosh monoblocks in the room right next door (a +100K setup). I walked from this room to the one with the KEFs and back again a few times. In some ways, I actually preferred the more realistic and unexaggerated soundstage of the KEFs and the midrange was just as eloquent. The sound of the KEFs is also not at all fatiguing and it was hard to stop listening to them.
However, if your budget is more limited but you are willing to upgrade your DAC as funds allow at a later stage, then, as the Stereophile measurements of the A2+ prove, the package that ridiculously outperforms for its price-point is the HD6. For nearfield listening at a computer desk, I cannot recommend the Audioengine HD6 package highly enough. No matter how expensive your main audio system, you should at least consider this for your computer desktop setup for nearfield listening (in itself a hybrid between a standard speaker based system and headphones). The price-conscious should do their budgeting carefully:
HD6 active speakers: $1000
AudioQuest Dragonfly Red $275
Audioengine S8 subwoofers: $510
TOTAL: $1785 (not including cables)
That's still about half the price of the LS50 Wireless speakers and you can upgrade bit by bit as funds allow. Change the DAC to a Schiitt Loki Modi Multibit ($399) and you add $124 to the price, just to give one example of an upgrade path. The iFi micro iDAC2 is $499 etc. It's still a reasonably priced package especially if spread out over time watching out for bargains with a bit of patience. I was so blown away by the sound after upgrading the DAC to the Dragonfly Red that I went and bought the Schiit Gungnir DAC! Yes, this is gross overkill. I snagged a good deal for a pair of RCA cables which, at full retail price, are worth more than the HD6 and contacted RK Cable to make up figure 8 power cables (Furutech terminations). Whether with the Dragonfly or the Gungnir, the sound, as with the KEFs, is one that you can easily live with. It is high-end sound on a budget.