If my head can do 4, 8, and 16 Ohms, how many ohms should a 2x12 speaker be?
it depends on if they are series or parallel. it also depends on what impedence the speakers are. so basicly, you can get a 2x12 in any of the three. find the speakers you like, and see what they give you and go with that. if your head lets you go with any of the three, do what you want.
What's the resistance of the speakers, and how are they wired (parallel or series)?
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^Yes, only b/c they have to match the ohm rating of the head to keep from destroying things.
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Well if my amp can do 4 8 and 16 which rating should I get for the 2x12?
If your head can do any of those, it'll probably sound very similar on each setting. Do you have any other dream heads in your future? I'd pick a rating based on that. For instance, Orange, Vox, and the ZVex Nano like 16 ohm cabs, while Mesa and a lot of boutique stuff seems to prefer 8 ohms.

OTOH, a cab wired for 16 ohms will probably also be able to be re-wired for 4 ohms (assuming two 8 ohm speakers). Making it a little more versatile than an 8 ohm cab that will probably only be able to be wired for 8 ohms (two 16 ohm speakers).

Helpful at all, or more confusing?
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I get it, also will this 240 watt 2x12 sound louder than say, a 140 watt cabinet, or is that just the limit for the head? My head is 120W tube, will it be alright?
That's just the limit, and it's fine. You could lose a speaker and run your amp full out into the remaining one. What kind of speakers are in that thing that are rated for 120W a piece?
You Don't Need a halfstack.

You Don't Need 100W.

Quote by jj1565
i love you slats.