I'd imagine also you may have a definition problem with just a single 15" speaker. You may already have enough volume, but certain frequencies just might not be cutting though. I wouldn't bother buying another amp without trying out different cab configs. first.
I'm jumping on the 'get an external 4x10/2x10 cab' bandwagon. Have a look around on the For Sale threads, there's bound to be something there that'll catch your eye. I'm pretty sure I saw a Mesa 2x10 floating around in there?
And yes, make sure it's 8ohm!
Truckstop