Q: I have heard that it is hard on a guitar amp to run into an impedance that is higher than the amp want to see. I always thought it was safe to run into a higher speaker impedance, versus running into a lower one.
A: The transformers and power tubes in a tube amp can be damaged by feeding too high of an impedance. Basically, the amp tries to deliver more and more voltage, until something breaks. There can also be problems with flyback voltages, which can be tremendously destructive to the amp.
Your best bet is to follow the manufacturers recommendations and ratings for matching speaker impedances to amps.