Bill makes an excellent point. Look at sensitivity ratings as well. Remember that for every plus or minus 3 db you've effectively doubled or halved the power of your amplifier.
380 watts should be enough for a lot of situations with an efficient cab.
A cab rated for around 400 watts or better should be fine.
An amp capable of overpowering your cab is good because if you're pushing your amp hard and distorting it you essentially send square waves to your speaker, which over heats the voice coils and ultimately fries your speakers. It doesn't take long, either. Speakers are more forgiving of too much clean power than they are of a distorting solid state power section. Tubes are a bit of a special case because they don't do the sudden start/stop of the voice coil the same way. The clipping is "softer".
Typically, cabs that are capable of more low end are less efficient (not as good at turning watts into decibels). A cab rated at +/-3 dB to 40 Hz will generally be less efficient than a cab rated +/-3 dB to 60 Hz. Unless you have a ton of watts (1000+) at your disposal you'll probably be better served giving up some of those super deep cabs for something more efficient. Those super lows are a mixed blessing anyway - they can really muddy up a mix in the room.
Speaker cone area is the most effective way to make your wattage go further. Unfortunately. A 15" speaker has roughly the same cone area as a 2x10. Two 12" speakers are roughly the same speaker cone area as three 10" speakers.
If you are looking at keeping your current amp (and why not? It's a good one) and you want to be fairly certain that you'll have enough power for most situations, I'd recommend a 4x10 or a 2x12.
If you want to hedge your bets, get an eight ohm cab. The eight ohm cab will only lose you 3 dB out of the chute. Noticeable, but not a deal breaker. This will leave the door open for you to add a second eight ohm cab later. Doubling the speaker cone area (all things being equal) will add 6 dB. Since you've also dropped your impedance to four ohms you'll get another 3 dB. You've just gained 9 dB or so. 10 dB is officially "double the volume".
All that having been said, for me the dream rig is probably the single channel Aguilar 500 watt head in a 1x12 "DB-112" combo with a DB-112 extension cab. In most situations I've been in a 4x10 or a 2x12 is enough and a 2x10 or 1x12 is often enough. My current rig is a 400 watt, 2x10 eight ohm combo amp. I'm going to build up a 2x12 eight ohm cab to push it over the top if necessary. If I had the parts for an eight ohm 1x12 or 2x10 cab instead I'd probably be fine with that.
Others will undoubtedly disagree with my summation.
KO
P.S. And as Bill (who has forgotten more about speakers than I'll ever know) said, go audition some speakers. And watch efficiency/sensitivity specs as you go.
