After some experimenting around and trying some differend Sub-Boxes if finally came to the point that my 5channel amp simply doesn't have enough power to drive the sub clean.

So I tried some amps and finally got a high-quality 4 channel amp. Now 2 channels are driving all speakers (1 channel left rear+front/1 channel right rear+front) and 2 channels bridged drive the sub. I've now mounted the sub-chassis into a closed box. I was told that it has the cleanest sound (but not less preasure than a bassreflex or bandpass).

I'm really happy with that solution - even though I can't change rear and front separately now - it sounds really good and clean now.

TeeMcBee
Got my Mk2! # 080000143