The typical advise when using a subwoofer with the 990 (or any other processor or receiver with good bass management) is to bypass the subwoofer's crossover, so you do not need your sub to have a built-in crossover.

The 7700 or 7500 could drive a passive subwoofer, but when building your own sub you'll want to figure out what sort of power requirements the sub may have. Subs often operate at a 4-ohm load, under which conditions you would be getting a pretty respectable 300W of output from either of those amps. DIY sub makers could probably offer some thoughts on whether that would be enough power for a specific design or not.