Amp's nominal power rating - any use?

As you point out perceived power is the result of a set of complex variables that would be impossible to reduce to a single formula. The amp output is one of these variables and as such has a value.

Just calculating perceived loudness:

(30W/8W)^(1/3.5)=1.459 30W is 45.9% louder than 8W but only in identical conditions. These power rating apply to max rms power and are pretty much useless with music. Peak power is more interesting since average music power is only a few percent of it (unless one listens to sine-waves). You don't really know how much of peak power each amp can deliver and it is not even possible to measure distortions at the peaks. Without measuring you perhaps push each amp to limits of audible distortions that are much less audible in even harmonics generating tube amp (even many percent) than odd harmonics generating SS amp (fraction of percent). Interaction with the speaker, being complex load, might also be different for tube and SS amps.

Isolated output power ratings hardly tell the whole story. You could buy a receiver rated at say 150 watts/channel and know it could`nt keep up with a Pass XA.5 30 Watt/channel at all. That Frankenstein is an 8 watt beast due primarily to excellent custom transformers and an enormous power supply, it uses the 6EM7 tube as a driver for the 300b(supplies a lot of current). A case of quaility over quanity.

Meant Pass XA30.5