need advice for sound improvement

True, but most speaker crossovers are designed to have the tweeter at ear height for optimum integration between the drivers. I would prioritize this variable.

Many thanks again to each contributor.
I received a lot of ideas and I will do my best to experiment most of them.
I am in the process of playing with the speaker placement and also I have ordered Jim Smith's book. Till it comes, I am watching the educational videos from the GIK Acoustics web site.
Will keep you informed once I have more progress.

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. 

Talk to GIK Acoustics.

End.

E

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.

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

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. 

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

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

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

If placement isnt helping any further then try new tubes.

10m speaker cable runs are very long. Try moving the speakers closer together. Put some cushions on floor near speaker ports.

Dear Helomech and dear Willemj

Thank you for your advises.
I will use them.
Currently the speakers are placed at 134 cm from the back wall and at 83 cm from the side walls. This location has improved the sound compared to earlier placements, but I feel that there should be "room" for even better.

That is a small room for this much bass output, and your description matches precisely the consequences (room modes). Moreover, transmission line speakers can have a lot of bass, but it sometimes is too much concentrated in a small part of the frequency response. If that peak and the room modes coincide, the problem can be even bigger.
So I would suggest you first measure in room response with a calibrated microphone like the UMIK-1 and the free REW software. Moving the speakers away a bit further from walls and corners may help. Depending on the sources that you us eyou can then start equlizing the reposne. If yo are using a computer as a source, there is the Equalizer Apo software. If you are using other sources, you will need to insert a Minidsp unit or similar.
As for the vocals, well maybe the singer was standing in the centre.

Try placing your speakers using the Rule of Thirds. You likely have some room nodes.

Download one of the speaker owners manuals from the Vandersteen website. They provide an excellent guide for optimum placement techniques that actually work!