I suggest researching this stuff. It's all over the place on the forums if you look around. I learned via Google. I won't be THAT guy who just says "Google it" though, so for a BASIC overview....
Ohms measure resistance / impedance. Speakers, like anything else in a circuit, resist current. Thus, you have 4ohm, 8ohm, 16ohm, etc. speakers/cabs. Most power amps deliver less power (measured in watts) to your cab as this impedance increases. This isn't necessarily a bad thing, as it can be used to your advantage due to the way circuits work. Throw two cabs with the same impedance together in parallel and the effective impedance of the circuit is cut in half (two 16ohm cabs in parallel = 8 ohms, etc). Thus, you can use a stack for large gigs, and just bring one cab for smaller gigs and not worry about the power amp blowing your speakers.
Heads don't have just one perfect cab for them. Experience, genre, etc. have a lot to do with it. You say 8x10", so there's a high chance you are into punk? To be perfectly honest, 8x10" cabs are a pain and a half to move. If you really dig the 8 speaker sound, you may want to look for a couple 4x10" cabs, both at 8ohms (so that when you stack them you run your head at 4ohms). Another option would be to go with a 6x10" cab at 4ohms. If you _have_ to have an 8x10", try to find a used Ampeg. Your head seems to be trying to emulate an Ampeg SVT, and punk sounds SO good through an Ampeg 8x10" rig
Some people will argue that speaker configurations shouldn't have a sound in and of itself though, and with that I encourage you to keep an open mind. I wasn't sure I'd like a 2x12", but it's worked out for me thus far (though not perfect). Through some playing experience, I have some ideas on where I want to go next. If you're anything like me, you'll probably end up with something else down the road anyway. Also, don't worry so much about your cab being able to handle way more than 550w (especially with 8x10", because a lot of those can handle a TON of power) because that whole underpowering thing is kinda a myth (how many times do those guys with 1500w amps use all 1500w all the time?). Your main concern is to have at least as much power handling as your head and hopefully at least a tad bit more to handle transients.
If you find a cab you're thinking about buying I encourage you to either PM me first or reply to this thread first. We have a lot of helpful folks around here
