Two if you can. Much easier to tune. I actually like 12 inch or smaller for better control. I use an Antimode Cinema for DSP control. Works very well and is only in the sub path.
Rel, Rhythm, and SVS appear to be the leading subs these days. Personally, I like low Q sealed subs. I think they blend better than ported and have less group delay. Even my HT, my twin 10 in push-pull sealed can make special effects move your stomach!
"Fast" is a bit if a bogus term used to describe other parameters. They have their frequency response and their Q. Both measurable. The sub, like ANY speaker just needs to follow the signal. No delay, with as little distortion, and as little overshoot as possible. A lot of sub "fast" is not crossing over your main speakers and sending too strong and too low of a signal to them. Boo-Kou distortion. Not having a subsonic filter on a ported sub can make a mush out of it too.
The magic of a sub in a Stereo is not just deeper, but cleaner. 802's go plenty deep. Surprised you need a sub at all. Not much music below 40 Hz.