Well, yes and no.
It helped considerably in my case.
Despite the attention given to room modes in the literature and internet chatter, modes were not the cause of the peaks and troughs I had in the low freqs in my room with my speakers. Instead my 'problems' were actually comb filtering (aka SBIR) rather than the standing waves of modes.
In short SBIR is the interaction of the direct wave with a strong reflection. If it is half a wavelength (or odd multiple thereof) out of phase there is a cancellation at that frequency.
Planar speakers have an equal amplitude and opposite polarity backwave, so when that interacts with the direct wave after a bounce or two off the front and back walls then a typical comb filtering response might be seen. The typical comb response graph can be obscured by other reflections from floor, ceiling and side walls. I used an Excel spreadsheet, room dimensions and some trigonometry to calculate path differences from speaker to chair via various wall reflection paths. These calculated path differences matched pretty well with many of the peaks and suck-outs.
So I moved both subs to unusual places around the room. The sub that I placed at the wall behind the chair (rear wall) filled in the suck-out that was caused be the Maggies bouncing off the rear wall back to the chair. Rear wall to chair = 2.5m ==> path difference = 5m ==> frequency of that as half wavelength (cancellation) = 35Hz-ish.
I place the mains and chair for the best soundstage, imaging, clarity and that sort of thing then place sub(s) to fill in the bass 'holes'. Sub positioning is never anywhere near where the mains are, but maybe that is just my room's geometry. My use of subs has nothing to do with bass reinforcement as such which seems to be the HT approach, rather a way to even out the bass in support of the room geometry and mains.
@Bilbo Hi Bill
I would think that your suck-outs at 150Hz and 180HZ would almost certainly be room geometry/positioning/reflection/SBIR. But what exactly is the culprit.
180Hz is approx a path difference of 0.95m. 150Hz is a bit longer at 1.14m. So the reflection from what surface/object is able to give a 1m-ish path difference.
Unlikely to be floor bounce. Floor bounce for a 3m listening position 1m off the floor is about 300Hz and higher freq for seats further away. Ceiling bounce is a possibility but only if the ceiling is 2.3m or less and even then planars don't have anything much off-axis to contribute to floor or ceiling bounce. Planars have a natural null at 90º and 270º so normally side walls don't matter much either, if at all. Its really all about the directwave and the reverse polarity backwave.
Hmm, so I would be looking at something roughly 500mm from the speaker or the mic that the positive direct wave might be reflecting from or something 1m away that the reverse polarity backwave can reflect off. [how far from the front wall are the bass panels?]
On a different tack but all effectively changing the direction of the first reflection of the backwave and the directwave.
1. You might try crossing the speaker axis in front of your chair. This has the effect of angling the back wave in a different direction and maybe reflecting off an entirely different set of walls.
2. Point them directly down the length of the room. Same reason.
3. Somewhere in between.
4. I tilt mine downward significantly, so that the top and bottom of the speaker are equidistant from the ears. Somewhat disconcerting to look at. They are very tall and my ears would be below half way were the speakers upright. However it's unlikely that such a small path difference between top and bottom would be anywhere near 180Hz. But it does change the path of the backwave and directwave.
5. I have the ribbon on the outside. Effectively the bass panels are close together and the backwave bounces off the front wall first and then off to the side, rather than the side wall first and back to the middle.
6. Unless the absorption is quite thick and plentiful it won't have much affect in the freqs that we are considering. Similarly dispersion is a mid-high freq thing unless the 'skylines' are enormously huge blocks.
7. Have you tried absorption immediately at the back of the speaker to kill the backwave as best you can to see if the cancellations are due to the backwave or not.
Having said all of that I am not sure that the 2 major suck-outs would be audible. There is a shallow dip from 60Hz to 200Hz, then a low hill from 200 to 1000Hz. I guess that this would give a mid range prominence to the sound, which is sort of what you describe. The overall look is not unlike mine but in my system I could hit that with a bit of gentle EQ. Maybe if you reduce 200 -1000Hz by a dB that will make 80 - 200 more wholesome. One of those supplied ceramic-resistor-things across the mid range panel might help. Are they meant for the mid range panel, I don't know; the sockets are there.
Tis all I can think of for now. A plan view of the room and positioning as suggested by Frank will help.