many fender ams will tolerate a 100% mismatch ...i.e an 8ohm load can go 4 or 16...Too risky to go 4 to 16 .If you go up you`ll lose power and punch ....some could actually prefer the softer vibe .
Don`t know what happens if you move down.
Depending on the circuit you could put too much strain on the tubes/tranny .In the case of the Champ ..nothing but 4 is acceptable (3.2 actually) .Surely there are more qualified people here to guide you , I just made the first post in an attempt to save your amp in case you were ready to wing it !
No, this would be for a single 10 or 12" cab. I have a DRRI for when I need the amp onstage with a band. This cab is to be isolated offstage to feed the PA and IEMs. The 8" Weber in it sounds good, but I'm afraid that I'm going to blow it to pieces eventually.