Resistances that are connected in series are simply added.

The more common connection is in parallel. The total resistance Rt is calculated as;

1/Rt = 1/R1 + 1/R2 + 1/R3 + ..... + 1/Rn

For example:

Two 8 ohm cabs in parallel gives you:

1/Rt = 1/8 + 1/8 = 1/4
Rt = [u]4[/u]

An 8 ohm cab in parallel with a 4 ohm cab gives you:

1/Rt = 1/8 + 1/4 = 3/8
Rt = 8/3 = [u]2 2/3[/u]

Series/parallel connection gets a bit tricky, but if you break it up into subsections and then combine them, it makes sense. For example:

A 4-speaker cab usually has 4 identical speakers in it, connected as two separate pairs in series, which are then connected together in parallel. Lets assume that all four of them are 8 ohm speakers.

So, for each pair:
8 + 8 = 16 (remember, [u]series[/u] resistances [u]add[/u])
connecting two 16 ohm pairs in parallel gives you:
1/Rt = 1/16 + 1/16 = 1/8
Rt = [u]8[/u]

Note that I use the word resistance here, instead of impedance. The two terms are not interchangeable. However, for estimating the net load a speaker setup will present to an amp, these calculations are close enough.

Matching the load to the output transformer setting is [u]very important[/u] for keeping a tube amp healthy. Since the Sunn 1200s has a transistor output stage, matching is much less critical, as long as the load is not less than 2 ohms. The above example of a 4 ohm and an 8 ohm cab in parallel come dangerously close to this value, but you can see that two 8s or an 8 and a 16 is just fine.