Thanks for the quick reply. Understood re: a voltage divider for the AKGs. Sounds like a good DIY project. For the selector, initially I was looking for a manual switch, but one with banana plugs. The best I found was a DIY job on another forum, so that might be fun to build. Until then, swapping plugs is no big deal.
I'm inclined to think it would make an excellent DIY project.
If you're doing it yourself, you can make a nice little box with bananas in and out, and multiple 1/4" outputs, including one with a voltage divider and one without.
Also note that all of our amps have protection circuitry that can be a bit fussy if you switch speakers with the power on - it tends to see a switch flip as a loose connection. (It won't hurt anything if the protection trips, but you'll have to turn it off and on again to reset it.)