Mono is usually better, but it doesn't really matter. main thing is that with a 2 channel, you ahve 2 channels and you have to make sure the gain's are set the same. You can run as many subs off of any amp as you want. You just need to watch impedence and wattage ratings.
class D amps (most mono amps are class D) are more efficient, and draw less current from your electrical system than class a/b amps. class D are also slightly less accurate, worse in SQ, than class a/b amps, but that's ok for subs b/c distortion is not as noticeable at low frequencies. not sure, but if you are asking if you can run your components off of that bd500.1, i would highly recommend not doing that b/c the distortion becomes very noticeable at high frequencies. if youre talking about running kicker comp subs, then he11 yeah.