Harald-
Re: the SW5500; As Chris mentioned, I use one now with my 5200 setup; Both new subs from M- (SW1600,SW5200) are marked improvements from the old M- subs IMO. No bottoming out as before and voiced much better to match the M- DSP line.

They have a tight bass as you would expect from M-, but also really put out when called on; They have more output than any other non-ported sub I've heard (I also like the Velodyne subs by the way);

One note of caution; the SW5500 is a bit overmatched to the 5200s I think; If you are into lots of LFE for movies - it's perfect; but if you're goal is to get the best pure music system - 2 SW1600's will match the 5200dsp setup the best. Of course that will cost significantly more money - M- prices the SW5500 very attractively in comparison. I actually used 2 of them for a while and got fantastic results for pure music applications - 80Hz crossover (or even higher) opened up the 5200s just enough and gave me what I could barely distinguish from the 7000 series sound.

Either way - I would recommend the new M- subs for serious consideration, right along with Velodyne and some others; After that, it's all up to your individual ears!!!

Best-

ken