I don't mean to be a wet blanket, but if the 1x15 combo isn't loud enough for your purposes, I'm not sure that pumping more wattage into a single 15 is going to be either. Sure, it'll be a little louder, but you can only get so much sound out of a single 15" speaker; that's your main limiting factor. I would think in terms of a pair of 1x15s or 2x10s, or possibly a pair of 1x12s.
Also, I don't understand why you've made up your mind that you want 15s and not 10s: There is no particular kind of sound that 15s make in general or that 10s make in general. Find a cab you like of either configuration and get two of them.
FWIW, the ideal rig to me is a pair of 2x10s in a tall stack, which not only is plenty loud but has many other advantages -- such as having some speakers up closer to your ears so you can hear yourself, and excellent horizontal dispersion across the room.
I can't believe it took this far into the thread to get to a voice of reason.
You aren't going to get a ton more volume from another 1-15. Period.
Watt ratings of cabs and even watt ratings of heads will not get you there. A single 15 can't handle all 500 ways anyway so more watts is not the solution.
You need more speaker surface area and better dispersion of your sound.
Two stacked 210 cabs is a great idea. It's modular so you can use only one for smaller gigs and home practice. It's easy to load in because most 210 cabs can be carried one in each hand. Get yourself a collapsible cart and you can throw your head and bass on there too.
But staying with one 15" speaker and expecting it to cover an entire room.... when your other single 15 couldn't..... is a recipe for disappointment.
Short version. You can pump a thousand watts into another 15. The only thi g that'll get you is a blown speaker. You need MORE speaker, not a different speaker.