Hi Troels,
I have just finished building a pair of Scanspeak subwoofers. Powered by Hypex FA-251. 
They are augmenting my already fantastic Illuminator-4 speakers - not to get more bass, but to mitigate room modes. 
The first front sub is crossed over at 55 Hz. The second sub, placed further back and to the left side of my listening position, is crossed over at 100 Hz. 
Careful placement, measurements with a calibrated microphone and listening made it all come into place. 
Not only did they iron out the room modes, but I also gained almost full level of bass down to 20 Hz compared to the 30 Hz I had in my room before. 
They have my full recommendation and I hope you post it on your website for inspiration. 
Thanks for yet another great design. 
Best Regards Michael