Although I love the 800, I am not sure anyone needs it. I have a 4,200 ft^3 A/V space open to the rest of the house and after calibration, the 800's gain control is at about 40% of max. This by the way is the factory default setting (is this coincidence?).
Slimpikins has a 10,000 ft^3 space and the 500 appears to be working well for him. I think the 600 is about as deep as anyone needs and the 500 may be good enough. Getting a couple of 500s may result in better bass for you if you care how it sounds to others besides you at the MLP.
BTW, with the 800, I've turned the bass volume up to about 70% and it remains tight and controlled with no audible distortion. This is certainly bass-head level in my room and I much prefer the calibrated setting. Ian has just finished the DSP code for a "gut punch" mode and I am looking forward to trying that out (see thread below). Ian is also looking at Air-enabling the DSP-based subs and including pre-built modes and getting rid of all the "hard" switches and dials on the amp. It would be cool too if we could create our own custom curves and even cooler if the sub could EQ itself via a microphone.
https://www.axiomaudio.com/boards/ubbthreads.php/topics/427740/'Nad_Tickler_or_Gut_Punch_-_it#Post427740
