Bass traps.

[Suedkiez]

There’s by the way a long discussion in this thread:

The Listening Room Reality

Don’t buy new speakers – The listening room reality I just bought a pair of speakers… fairly expensive ones, in fact. Nothing really interesting. The point is: was it money well spent or just a waste? Sounds like a silly interrogation, doesn’t it? In fact, it’s not, not at all. Do you really think it is your speakers you’re listening to? Here is a nice video (about 30 minutes) which tackles a reality not always understood, or ignored, by audiophiles. There are loads of videos of that kind…