Thanks for your answer!

The fluttering must come from inside the box. Can´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 it´s very deep for the twins, but for example my NS-10 just cut off what they cannot image. There´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 aren´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 it´s usual?

Thanks!!