Get rid of excessive low bass

Detlof is right. A square room is likely to give you terrible standing waves. You might try firing the speakers in a diagonal fashion. Room treatment (bass traps will help), but a 4 meter room is going to give a bass boost around 43 Hz. Most room treatment doesn't work very well below 80 Hz. The only other alternative is to go to an active equalizer. Audiogoner's will accuse me of Heresy--but unless you can change the room's dimensions you need a notch filter around 43 Hz. If you can afford something like a TACT I hear they are excellent. If you can't, I would suggest a McIntosh EQ. Unfortunately, it is analog and in the signal path and will add noise--a small amount. The McIntosh EQ's are very good--but not perfect.

abstract7, no heresy charges here! :>) yure absolutely right - bass traps or system-equalization will be the simplest, most effective cure.

re: moving the speakers around - try off-setting the speakers a bit - i.e.: have the left speaker either closer, or farther from its nearest side-wall, than the right speaker... or, even cock the entire plane of the speakers - i.e.: have the speakers different distances from the rear-wall, even tho the speakers-to-listener triangle set-up remains the same.

try elevating the speakers a bit as well as trying angled off center placement as suggested above. I would also experiment with placing furniture (or other objects in the corners both at the floor and ceiling.

Just recently I have been playing around with cords and cheap ($500) powerline conditioners. During that experimentation I found some interesting things could happen
One: rum a really small cord to your amp... 18 gage or less, it will cut your low frequencies and make them seem thinner but clearer.and the rest of the material sounds cleaner. A balance between too lean and normal is just some experimenting away... even a section of narrow wire in the powercord line... The effect is as if before: a thick woolly rinocerous was in the room: add the skiny wire and the bass becomes unwooly, cleaner... more angular. And cheaper than to buy a crummy EQ box.
Also try padding up one side/corner with drapes, heavy throw pillows in the corner angles on the floor.

I have a Tact 2.0 digital processor and confirm that it does a magical job of cleaning up the bass response in a room. When I swith the unit to "bypass" the bottom end becomes unbearably boomy. It's hard to believe that in pre-Tact days I tolerated this sound. Another less expensive alternative would be to find a Legacy Steradian processor. It is a very specific kind of analog equalizer, being an analog bass notch made specifically to tame room boom. One is able to adjust the depth of the notch, I believe. And as far as using power cords to equalize a system... I won't argue with those who have had success, but it's rather Rube-Golberg-esque way to equalize a system, don'cha think? Why not address it using tools intended for the job (i.e., digital or analog equalizer)?