I agree with Seth, I would not use the A500 as a sub amp. I'd actually look at good plate amps rather than a separate amp, unless you are trying to keep the sub and amp separate. Sub amps tend to be designed with that type of abuse in mind. A crossover is really only a benefit if you plan to use it; if you are feeding the sub from an already crossed over signal, you don't really need it. All amps should have gain control, but phase might be a plus and one thing that you generally don't see on separate amps that you will find on better plate amps is a subsonic filter to protect your sub from lows outside the range of the design - this helps with power handling and helps to prevent bottoming.