RMS is the base line everyone should use. Back in the '60s and '70s, cheap stereo gear was sold using the ludicrously inflated "music power" and "peak music power" wattage figures. They have nothing to do with reality. Music power is twice RMS, and pek music power is twice that. RMS is based on Ohm's Law. Music power and peak music power is based on salesman speak---i.e., marketing. Caveat Emptor, my friends! HCSterg has it right. I'd also suggest that the class of amp makes a difference---A, AB, D, etc. How does a class A Vox AC30 keep up apparent SPL with a Fender Twin class AB at 85 watts? Solid state amps can't really compare with tubes at equal output levels, and they still haven't caught up in terms of tone. A simple tube amp like a Fender tweed oozes tone naturally, where the highly involved circuitry of a modelling solid state amp can't get close. They're getting better, but still have a ways to go.