Building a 212 Speaker Cabinet

You could also do a stereo/mono switch, which would give you the ability to run the whole cab on one amp at one impedance, or each separate speaker (stereo, or one by itself) on it's own at another impedance. Speaking of which, anyone know where I can find such a thing for MY 2x12 cab???

In case you haven't learned it already, it is generally assumed that running two speakers in parallel will sound better than running two speakers in series. I don't remember the ultimate reason for this, but it is fairly agreed-upon. So keep that in mind as you figure out your speaker choices. I assume you DO know that wiring two speakers in parallel will drop their collective impedance (ohms) to half of each speaker's. I.E. Two 8-ohm speakers in parallel will be a 4-ohm load, two 16-ohm speakers make an 8-ohm load, and so on.

Yeah, general rule of thumb is that it is BEST to run matching impedance numbers, so you would be BEST to run a 4-ohm cab with your 4-ohm amp output.

However, this is the real world, so we all have to run some mis-matched impedances from time to time. You can generally run a cabinet with an impedance number HIGHER than the amp, although I'd keep it within one level (impedances generally are multiples of 2, so 2-ohm is followed by 4-ohm, which is followed by 8-ohm, then 16-ohm, and finally 32-ohm). I wouldn't run a 16-ohm cabinet with a 4-ohm amp, but so far as I know, this isn't going to blow your amp up. With your 4-ohm amp, an 8-ohm cab is going to be safe.

However, you NEVER want to run it in the reverse, where the speaker cab is a LOWER impedance than the amp's rated output. So, in your case, DO NOT run a 2-ohm cabinet off your amp's 4-ohm load. This is creating a circuit that is begging to short. This can and will damage an amp, in most cases fatally.

When running a higher-impedance cabinet than your amp, the amp essentially has to work harder to overcome a higher impedance, which means that the amp's output in watts will be reduced when running this way. This is in no way a guarantee, but you would lose approximately 20-30% of your amp's output power by running a 4-ohm amp into an 8-ohm cab.

Running an 8-ohm cab off your 4-ohm amp will cause some tonal coloration, but it will be fairly negligible. Some folks can probably hear it, but it's not a HUGE change in tone. The biggest effect will be a slight loss of higher frequencies.