Hi Orthopod,
I'd like to start with segregating the problems:
1.
Center speaker placed low. This is less than ideal, Tilt the speaker up so that the tweeters face you at the spot where you sit.
2.
Plenty of reflecting surfaces. You're correct in wanting to apply absorbers, but bear in mind that these will not help for low frequencies. But do provide absorption so that the mids and highs don't sound harsh. Auditory fatigue will set in after an hour or so of listening, and you might just come out of your HT carrying a little headache, not necessarily caused by a bad movie
. I'm not in favour of glasswool either, and I always prefer woodwool boards, and some other eco-friendly fillings depending on the kind of finish you have in mind. With woodwool, fabric will be your only option for a finish. Anutone is fairly the most expensive option available, and woodwool alone costs a lot less. If you wish to use other eco-friendly fillings, try to use a metal grid for stuffing, that way the sawdust and carpentry will be avoided.
3.
Poor bass response - perceivable peaks and dips, empty spots and booms. This is fairly predictable in smaller rooms. This is also the most tricky, and I don't suggest any kind of trial and error ( like "trying" to add another sub). For accurate treatment, room modes have to be calculated - these are resonant frequencies unique to each room. The dimensions of each room causes some sound waves to form exact multiples of themselves, and this causes stationary peaks and dips. To a lay person, this will mean an obvious dip or rise in boominess at various spots, instead of the uniform, tight-sounding bass that you wish to hear. Bass traps are usually calculated and designed, and they must be placed at specific problem locations in the room. I do not advocate randomly placing bass traps - it is a shot in the dark that leaves you poorer on effort and money, and on aesthetics.
For home theaters, a lot of times there are just no problems and your listening experience leaves you happy. However, in the event of you actually facing issues, the acoustics are enough to upset your experience. Having spent so much on the equipment and in getting the room done, I'd advise you to get correct technical advice from a local acoustical consultant.
This is pure science - the location of every peak and dip should be precisely calculated, and you won't run the risk of excessively treating the room and rendering it dead. I mention excessive treatment because uniformly applying 6" thick treatment on your walls will cause the mids and highs to be over-absorbed, and the room will not sound "live" anymore. While 6" of treatment may even work for the woofer frequencies, it is virtually transparent to the subwoofer frequencies, which systems these days can produce. Only when things don't fall in place by themselves, is accurate treatment more helpful than guesswork.
All the best.
