mbz

A good structured approach, good luck with your search. Might need to refine your search a little depending on what era (vintage?) of equipment you are interested. Most vintage will be discrete maybe throw in the odd opamp or display driver... However AVR's and the like will be microprocessor controlled with a fair sprinkling of other IC's. Generally the AVR's are a difficult repair. Important to be able to follow the schematic, many freely available on hifiengine.

Kudos for taking up the challenge/in search of the truth. Without such an approach we would still be "knowing" that stomach ulcers were caused by gastic/stomach acids(?) Doubtful they will give you a Nobel prize for you findings on ethernet cables... At the moment I'm yes to audio grade spkr and interconnect cables. Skeptical about shielded power cords for $$$ and "power filters" for $$ or $$$. Open to fully reconstructed sine wave/power, sadly prohibitive at $$$$'s

Depending on budget, vintage might be a good option for vinyl, giving you that "warm sound". Something from Luxman (SQ-505X,,,,L-30/31) or Sansui (AU-505/555A and plenty of receivers). A dedicated phono stage (Vincent PHO-8 and others, again on a tight budget...). You could get a refurb'd amp for about $5-600??. Plenty of used gear (see SNA classifieds) if you've got more to spend...

Regarding the bias issue, might be that there is an issue, it's a simple amp so the bias should be in the ballpark unless something is notok. Suggest carefully check the dc voltages at the base of drivers and outputs, eg, TR805, 807, 809, 811

Typo, transistor is 2SC871, KSC1845 may be suitable sub, will check datasheets. Datasheet I was looking at suggests 2SC871 is bce while KSC1845 is ecb, otherwise appears to be an ok sub, double check the pinout of the installed 2SC871, ie, trace tracks/components.

My starting point would be to use the trusty tektronix CRO. If the Vdc is not detectable by the CRO then I would research "best practices" for measuring low Vdc on mains and consider building/purchasing necessary test equipment

No suggestion the item is DIY. I have a small "hi-fi" budget so need to spend wisely. The item pictured is a bit expensive though I understand product development, short production runs, warranty, sales and profit margins. Keep in mind such items should only be considered if you have a Vdc issue and not as a upgrade path for SQ. First step for me will be to check for mains Vdc and go from there. As an engineer I'm comfortable working with 240Vac, thanks for your concern. Others, keep in mind 240Vac can be lethal.

Think I'll diy and "see" if I can hear any improvement before handing over the $$$'s for the assorted bells and whistles.

Your choice, either the economy or the isolated, the links are the correct items. The "problem" with the economy is that in the rare event there is a winding fault then full mains AC (240Vac) MAY be presented to your USA appliance(120Vac) which will probably kill it. Question then becomes how valuable is the appliance, in this case I'd say not very valuable so the economy is an ok choice. If you were using it for a mint vintage Sansui aplha... or similiar then I'd argue strongly for the isolated.

Take a look inside... Caps are maybe $10- each, the meters are probably the most expensive item in the box... $550 yikes...

Just to be clear, you need a stepdown transformer, from AUS 240Vac to USA 120Vac. Tortech are suitable, https://www.tortech.com.au/product/voltage-converters/step-down/isolated/isolated-american-step-down-transformer/ They make 2 types, isolated (safer but more expensive) and economy. 250W unit is listed at $110- while the 500W is about $150-. Need to check the power consumption on the rear of the unit, 150W?? Also need to workout likelyhood of you buying other 120Vac equipment, so consider getting a larger unit.

I'm not into phono/vinyl (can't afford it) so I may be off the mark a little. Switch the phono RCA's, ie, TT-Left plug into Right Phono RCA, likewise TT-Right plug into Left Phono RCA, expect hum to change channels. Check the 4 wires that connect from the tone arm to the cartridge. 2 of the wires are for audio (L&R, white&red?) and the other 2 are earths(blue&green). Often they connect as a slide on "clamp", check connection of the green wire.

Danes are always welcome, provided so much good hi-fi over the years...

Problem the same after switching spkr cables rules out source and amp. Have you physically swapped spkrs? Maybe use REW(Room Equalization Wizard) need a mic also take some time to learn how to use SW...

+1 on spkr stands/tweeters at ear level when seated in normal listening position. The AU-555A is a very nice amp.