There’ve been some threads on this subject, and I’ve posted some downright confused replies (depending on wine consumption etc). I’ve checked with others, and here is the consensus for VALVE amps. Don’t ask me about SS 😳

Obviously, match your 8ohm tap to your 8ohm cab (or whatever) if possible. For those who don’t/can’t; here is the definitive better/worse:

BETTER: The tap (or selector) is LOWER than your cab. eg, 4ohm tap to 8ohm cab; 8ohm tap to 16ohm cab. You will overwork things but…

WORSE: The tap/selector/output jack is HIGHER value than your cab. ie, 16ohm tap to 8ohm cab, or 8ohm tap to 4ohm cab is not so good. This really puts a strain on things.

Having said that, you can get away with most mismatches if we’re talking a x2 or a /2 mismatch. 4ohms output to 16ohm cab (or vice versa) is asking for more trouble. Hope this helps, but I’ll expand if possible. Better still, a real transformer-winder will appear and save me…