Furnishings will help, but if you do the clap test, and hear echoes (as you may have described above) then you may want to refer to this thread:
Sirquacks Bass Traps It might help, in your situation, may not...but different rooms produce different results. Also, have you calibrated the speakers via an SPL meter, so that all speakers are in 'sync' per se?? Might try that, with an SPL meter, from radio shack, or otherwise.
As far as the sub goes...have you performed the 'sub crawl' yet? Basically, put the sub in the primary listening position, and crawl around the room (walls) to find the spot where the bass sounds the best. This is where the sub should go...and trust me, it works wonders.
Good luck.
