need advice for sound improvement

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.

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.

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

If placement isnt helping any further then try new tubes.

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