Thanks for your answer!
The fluttering must come from inside the box. CanΒ΄t be the room...
I attached a short video and a wave file to show it better...the kick is very low (peaks at 30-50hz). I know itΒ΄s very deep for the twins, but for example my NS-10 just cut off what they cannot image. ThereΒ΄s no fluttering at all on those...
The movie is recorded with my mobilphone. It cuts out all the depths so the fluttering can be heared clearly. My monitor controller shows about -9db at -10dBV input setting on the focals. As you can see the woofers arenΒ΄t overstressed too much.
Maybe someone is able to play this file on his twins at a similar volume...I would really like to know if itΒ΄s usual?
Thanks!!