On the 8x8 for example, how do you switch from 1 cab to 2 cabs for a head? You rotate the dial but you still have to change the impedance from your head right? If you set everything up for 16 ohm then run 2 cabs, you have to get behind it anyways.
I want one of these, whatever brand, but Im hesitating because it seems like I need so many cables and still need to get behind the amps.
That is one of the limitations of these switchers. If you do come across scenarios where there will be a mismatch you'll need to figure out how you want to deal with it. For instance, if you have 2 heads; one at 8 ohms and the other at 16, and a 16 ohm cab. Or as you mentioned using a switcher capable of running 2 cabs at once.
There are a couple of ways to approach this depending on which scenario you're dealing with...
1. Deal with reaching behind the amp to flick a switch and change impedance.
2. Have your setup where everything is on the same ohms no matter what.
2a. All amps and cabs are the same ohms and only use 1 cab at a time.
2b. Select only the combinations where ohms will be matched.
2c. Use amps with one jack that can automatically do 8-16 ohms
3. Use an impedance matcher like a Webber Z-Matcher or an attenuator that has impedance select like the Fryette Power Station.
4. Controversial, but not worry about the mismatch as long as it's no more than one step in either direction; 8 to 16 or 16 to 8.
4a. Several reputable sources such as Mr. Steven Fryette say there are no issues with a 1 step mismatch.
For me personally, all of my amps and cabs are able to be setup with no ohm mismatches and I have no need to run 2 cabs at once since everything I have is a 412. My switcher is an 8x8 deluxe so can select only one cab at a time anyway. If I did come to a mismatch in general I wouldn't worry about it and just roll with it. Or if I did have a concern use my power station to compensate.
Yes, there are a lot of cables involved in the switching setup. But if you think about how much money you have invested in amps and cabs to the point you need a switcher then the little bit of extra investment in cables will be negligible.
Just put some good thought into what you want to accomplish with the set up and how best to do it. Do you need to run both cabs at once or will you always be doing that? you may need to make and be okay with a compromise or two. If you need some help with that let us know what you have. I'm sure several can chime in to help you figure out the best way to work with everything.