I was about to purchase the F12G but wasn't impressed with Enrico's unwillingness to entertain a call. He only wanted to communicate via email. So I did some additional research and found the SVS SB-4000. Apparently this sub pairs nicely with Buchardt's S400. Please vote and or provide comments. Thank you so much 
How is the M10 working out for you? Have you had any issues with the software? I'm waiting for M33 to be released, I will be waiting until NAD gets most of the bugs fixed. Apperently the 778T once you turn it off the first time, if won't turn on the next day and USB doesn't work, reset doesn't work it's just completely dead.
I haven't heard the SB4000 but I have heard other subs from SVS that are smaller. Make sure you are able to adjust the crossover.