I think the p1000 is a fantastic cabinet. I use mine with a GK 1001rb-II head. In combination with my Epic, I think the sound is heavy, powerful, and pretty clear; though what bkbass said about the slight sloppiness and sag is true. I used the sound to my advantage, playing in real aggressive bands and matching Mesa Dual and Triple Rectifiers pound for pound, so to speak.
I bought mine about 13 years ago, however, and I know or at least have heard that they've changed many things (speakers, design) about the Powerhouse cabs since then. I don't know what they sound like anymore. And they *are* heavy, as in weight. But, if you're interested in playing through one, I'd take your amp head and bass to the store and see if you test it that way. That's what I did, so that I knew what I was getting into.
Good luck!