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.

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.

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.