That amp should be sufficient, but is probably not nearly what the Plus can take. If you wanted to step up, I'd say sell it and pick up one of the 500w BASH amps, probably the one with the PEQ built in, which are probably closer to what is used on the PC Plus models (525w).
If it is the amp I am thinking of, I am using a similar one, one model up from that that produces 350w from the same manufacturer and it is driving my 15" with no problems at all; quite impressive actually. They are more potent than they seem, but still not going to be the same as the amp that comes in the PC Plus.
I'm not sure about biamping a dual voice coil driver. I don't think you can do that because it is one circuit.
That sub is not overkill for that room, IMO.