need advice for sound improvement

Thank you everybody for the new hints. I will try to experiment each of them.
To shorten the cable length one of the cables has to run across the walking path as the back wall of the speakers has a circulation door placed with its axis at 2/3 of the wall length. This room is also our living room and we have a dog pet which would sometimes interested to play with this cable. Between the speakers and the back wall I built a piece of furniture which goes from side wall to side wall and above the door up to the ceiling. The electronics are sitting on shelves on the narrower side of the door, while the TV set is placed on a shelf on the larger side of the door. Therefore the R speaker is at about 1,5 m from the power amp terminal while for the L speaker I ran the cable up the wall, passed on top of the door till the other side wall and came down and along the floor to the speaker. That required about 10 m. This solved the aesthetics and the practicality of day to day living but definitely ruined my listening experience. Moreover, this door as well as the door from the R speaker side are always open as there is a frequent traffic. 

The 10 meter speaker cables aren't the problem.  I have been using 8 meter speaker cables for 10+ years with no side effects.

The speaker cables should indeed not be a problem, unless the cables would be very thin. See here for some data: http://www.audioholics.com/audio-video-cables/speaker-cable-gauge In general, if a speaker cable has to be long, the solution is simple: use a thicker gauge. In your case I doubt this will make any difference.
The most obvious explanation is still room modes, perhaps aggravated by the transmission line design of the speakers.

Talk to GIK Acoustics.

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I built a modified QRD 13 that's 20" deep, with the verticals 3.5" wide. It broadened my soundstage and helped my bass. I placed it flush with my front wall. Hemholtz absorbers may work, or Dennis Foley has plans for an absorber I'm going to try. Multiple subs will also help. I have about 20 wall panels made of 1" compressed fiberglass board covered with speaker cloth that really helped me.