Are you talking about replacing the PB12, or adding a second? If you get a second, get a second PB12, as it's best to have matched subs. Either that or sell the PB12 and get two EP500s. Otherwise, you have two different EQ curves, and that can really wreak havok on your room response.
Generally, a second sub will help level out room response so that you don't have a ton of bass in one spot, and hardly any bass just 3 feet away. I've played test tones through my sub while walking my room, and it's amazing how in one spot you can get a huge amount of bass, and in another location in the room, you can barely hear anything.
A second sub will also give you 6db more bass, but it sounds like your first PB12 isn't running out of gas. So likely what you would end up doing is turning them both down a little. So your main benefit from the second sub would be to level out your room's bass response.
If your room's bass response isn't really concerning you right now, getting a couple M80s and a VP150 would definitely be a nice option. But it won't really do anything for the bass, unless you cross them over at something like 40 or 50Hz. Generally, 80Hz is what everybody goes with, but the M80s are bass capable speakers, and you may well have better results crossing them over lower. It's all about trying different things and seeing what sounds best to you (and your wife).
Good luck!
