floorstanders on carpet over concrete

If you speakers have ports try stuffing them or covering them ,that would change the sound.

have you stuffed the ports to dampen the bass a bit? try 1/4 thick foam rolled up to fit the diam. of the port or ports. try different lengths and or thicknesses. you never know, worked for me.

I would try tube traps or echo busters. Go to there web sites as you can get great info for this. Room acoustics play a huge role in sound so you need to get educated about this. There are many types of room treatments that dont look half bad and will blend with room decor. I used Bass Busters, Echo Busters and Sonex Pyramids all improved my room by a bunch!
Happy Tunning!

One comment about my speakers placement- the basement is inverse L-shaped with the listening room occupying short part (17x21 ft.),which places the right speaker near the long wall, with the open space to the left of the left speaker. The distance from the short back wall is appr. 4ft., the distance b/w speakers is 8-9 ft., so is the distance from the listening position. If I'll decide to go the bass trap route, what would be your reccommendations for the traps placement.

Try adding some more cushy furniture in the room. It's free and worth a try if you can move something down there easily.

Try moving the speakers away from the back wall three inches at a time until the bass firms up. You may end up with them to far into the room so bass traps will be needed. When I ran my dedicated lines the most improvement was bass then detail. As stated above by moving breaker to other side can only effect eq. if the other appliance is running at the same time unless its a frig or something that is on all the time. The most important time to have breaker on same side is when multi dedicated lines are used, you want them on the same phase or leg of the sub panel. In my HT set up I run Paradigms and I find bass to be some what muddy, not as fast as I would like so that's where the sub comes into. I find these speakers need alot of room behind them. I think your best solution will be the bass traps! Another thing get rid of the crappy spikes that came with your speakers and pick some Michael Green Audio Points up as they worked for me. Good Luck Go to my system and scroll down to see my dedicated line project.

I don't use any Power Line conditioning now, only a dedicated lines. Yesterday I tried to switch to my "old" lines, following the Tireguy's advise- didn't make any difference bass-wise.

You might try a set of Aurios Pros under the speaker spikes, four to a speaker. They will certainly isolate the speakers from the concrete floor, and clean up the bass. Not inexpensive, but what tweak is these days? Audionuts is one dealer which carries them.

I think you could try slabs under the speakers but that doesn't usually remedy too much, more of a tweek then a tuning device. I think your problem could be room related and bass traps are an option, helmholtz resonator's(aka room lens) could work as well; different rooms, systems and listening preferences have different requirements. You also may want to try moving your speakers a little bit, spiking shouldn't make them unlistenable. Also if this problem happened only after the new dedicated outlets, you could have taken power from the 'dirty' side of the box- the side with the washing machine, refrigerator, etc... a simple fix may be to move the breakers to the other side of the box and possibly replace the grounding rod you are currently using. I would try plugging your gear into different outlets first, if the problem seems to go away it is not room related(entirely) but if it remains then you have a good place to start. If the new lines are the problem then you should track down the problem with them and it should be fairly simple to fix. What ever the case is this doesn't sound like it should be keeping you up during the nights, get things back to normal and enjoy the music.