The 100/60 mismatch is significant but keep this in mind. Amplifiers don't produce full power until they are turned up quite a bit. The point on the knob depends a great deal on the input signal from the guitar/fx chain. Low output single coils might not drive the amp to full power until the master is at ...say 8 where a high output humbucker might see full power at around...say 6. Obviously these numbers are just for example. Next thing to keep in mind that the difference in volume out of the speaker between an average of 100 watts and an average of 50 watts isn't very significant. It's only about 3 decibels or "a little bit louder." So as long as long as your not cranking the amp to almost full volume your really not getting close to that 100 watt figure. A speaker in real distress will be pretty obvious and you'll most likely have plenty of warning that you're giving it too much juice and time to turn down. I would have no problem running a 100 watt amp into a 60 watt speaker...just watch the throttle.