Listening Room Upgrade

I am having the carpeting replaced in my listening room and am looking for any ideas on how to improve the acoustic properties of the room while I am at it. The only ideas I have come up with are to put down 1/4" plywood flooring to stiffen the floor and to use non-synthetic carpet. Any help with material choices and installation methods is greatly appreciated. My current floor consists of modern floor trusses(18" O.C.) with 3/4" tongue and groove flooring. I do not think I can use a material greater than 1/4" for the sub-floor because of matching considerations with an adjoining room. God bless.
I may get some negative feedback for this, but I like hardwood floors with area rugs. The reason for this is to let the speakers sound bounce off the floor in the nearfield, but then get absorbed further into the room. I have done this in a few listening rooms with very good results. In my opinion in helps create a soundstage--now it won't work for all speakers, so I should qualify that I have done this successfully with Martin Logan speakers (which as most know are very finicky about placement, and have to have a reflective wall behind them to create a good soundstage to begin with). As for the carpet, a good thick wool oriental works great.
i agree with abstract7. you might better spend your money by placing more cross-bracing underneath your new floors, replicating 10-12" o.c. if you do install wall-to-wall carpeting, i'd suggest you cover the area between your speakers (outside edge to outside edge) with a thick wool area rug or "oriental carpet." this solution works better, in my experience, than using a natural fiber carpet alone. -kelly
In addition to the above suggestions... If you want to reproduce the lowest bass octaves, stiffen your floor from below. If you have heavy speakers, seperating them from the overall cross bracing and bracing them to the sub floor in the basement or crawl space also helps. Treating first reflections with something like ASC tube traps gives a significant improvement in the mid bass to midrange.
There is a finished room below the listening room precluding any modifications from the under side.
If you go with wall to wall carpeting make sure you use a thick pad.
Various carpet types have different absorption coefficients throughout the frequency range. A thick pile carpet will absorb more sound than a thin one. What makes even more difference though is the type of padding you use. For best acoustic properties and a more even absorption extending down to lower frequencies, use a hair-felt padding. This is kinda an old-fashioned padding, most of the stuff today is foam of some kind, but if you can find it, the hair-felt would be the best. As your second choice, use a foam with open cells - like foam rubber. A padding with closed cells or a combination of closed and open cells like sponge rubber won't be quite as good.

One of the earlier suggestions was to keep the wood floors and use an area rug - which could also be a good option if you do other acoustic treatments in the room. The problem with carpet is that it absorbs frequencies above 1kHz pretty well, but doesn't work very well especially below 500Hz, so you get some uneven absorption across the frequency range. You can even it out by adding the additional plywood to the floor like you were suggesting, and perhaps also adding a few bass traps. Or as suggested before, you could keep the wood floors, and add an area rug between the speakers and your listening chair to catch the first reflection from the floor, and then add the acoustic panels to the side walls, and on the wall behind the speakers. Without the wall to wall carpet, you will need more panels than if you had the carpet. Either catching the first reflection from floors, walls and ceiling can really focus the sound and improve the soundstage.
"There is a finished room below the listening room precluding any modifications from the underside."

I agree with the others about more bracing under the floor. Have at that ceiling below. Drywallers are quick and cheap. This is your stereo we're talking about!!!