I have a Traynor YCS-100H amp head that has a 100W mode and a 30W mode. It also has a switch for impedance, so it looks like I can choose between 2 or 4 ohms in 30w mode and 4 or 8 ohms in 100w mode. I recently bought a used 2x12 speaker cab to run this amp through, and I'm a little confused by all the options. When I asked the seller about the impedance of the cab, he told me that "The speakers are both 8ohm, wired to 4ohms. You can rewire them to 16ohms if you wish."

I don't want to run the amp improperly, because I know that sending the wrong load to the speaker could damage the output transformer.

So my question is this: which mode(s) can I use on this head? If the speakers are wired to 4 ohms, does that mean that I can only run the head at either 30w or 100w? Could I run its at 2 ohms instead of 4? What would be the difference if I did?

I know that these are newbie questions, but I've only used combos in the past and I don't want to damage the amp by running it improperly. Thanks.

Here is a pic of the rear chassis of this amp:
Your current cab is wired to 4 ohms and that is all you need to worry right now.

So pick your wattage and then pick 4 ohms.

If you want to rewire the speakers you can rewire your cab for 16 if you wanted to. Given your amps power options now, I'd just leave it at 4.

another way of saying it:
2 x 8 wired in parallel gives you 4
2 x 8 wired in series gives you 16