Scansonic mb serie

I think some music is recorded to sound that way. I have some Chesky demos for speaker placement that sound like that. My Dyn C1's did it too. But my old Mission/Cyrus 782's never did sound like that. OTOH your room and seating position will contribute  to sounds behind you. But extreme right and left IMO is a sign of a really well designed speaker.

BTW the X/T -1's are a much higher class of speaker.

I heard the MB-6 at dealer it had some bass boom, powered with Avaik electronics

This thread is very interesting. My MB-1 driven by vintage Mcintosh integrated/separates do not sound booming in bass at all. I actually use them with a subwoofer. They sound pretty sweet when paired with the warmth of the Mcintosh. 

Just to say that at the Sound & Vision Bristol HiFi show I heard Scansonic M6 floorstander (at the time price was £1000.00 now understand it has gone up) what immediately struck me was their resolution, airy sort of 3d presentation and realistic sound.  At the time blues music was being played and these slim floorstanders made a deep impression on me espescially the musical slam these speakers were capable of producing.  However as I read Scansonic speakers require a long burn in and are a little fussy as to placement.  They are not so popular in UK and one dealer stopped stocking the brand as there was no demand. Its a shame if only these speakers required little warm up and were room friendly!