The best advice here is in post #20 from 'gustav' otherwise it's just guessing. The soft furnishings you describe do not really represent room treatment though, as they are narrow-band absorbers and as such will help in the high frequencies only.
Measuring your room will allow you to see on a graph exactly where the problems are. You may have cancellation issues causing this.
My suggestion is to listen to your system as it is on closed headphones which will take, for this purpose, the room essentially out of the equation. If the 'missing mid range' is back to entertain then what could be the problem? ... it's the room! If the problem remains then start the laborious process of elimination.