need advice for sound improvement

Buy Jim Smith's book Get Better Sound.
Read it.
Read it again.
Follow his instructions.
Get your setup correct.
Install acoustic treatment.
Sit back and be amazed how much better your current system now sounds.

The room dictates the sound. No gear can overcome that. You have to get the setup correct. No shortcuts.

Tom

FWIW, I use and recommend acoustic panels from Primacoustic, purchased from Sweetwater Sound. Fantastic customer service, highly recommended. 
I'm not an owner, I'm just a customer. A very happy customer.

Tom

+1 10m speaker cables are crazy long shortening them and adjusting speaker positioning should help immensely.

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.