I have an 11' x 11' room and big speakers. I have seen differences in the Preamp and the soundstage. Also the recording itself make a big difference. I have bad vinyl and bad CD's. As well, the reverse. The best I had was using bass traps in all corners, but my wife made me remove them because they were "Ugly". I tried to pain them white, a very bad idea. Buy white ones, they are said to yellow after a time, but still my recommendation.
Got an Anthem STR Preamp that has ARC, (Room Correction), not set up yet, but my next attempt.
Another question is, are the polarities of the cables all correct?