Amp's nominal power rating - any use?

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.

Its probably the best reasonable rating anybody could come up with without getting to much into specs that are hard for a lot of people to understand.If they give it at full bandwidth,it sure helps.The speaker being driven creates a hard power rating spec for a one fits all amps.Their impedance,(speakers resistance) is always varying making power ratings tough for amps rating.Amp matching with speakers can be more critical than watts.Speaker efficiency is very important also.Doing some research on whether your amp will drive xyz speakers can be more useful,especially if they have low impedance dips.

I have no idea what Kijanki is talking about, with his percentages.

Your speakers are good for about 87db for 1 watt into 8 ohms (at one meter). This means an amp that clips at 8 watts will drive the speaker to 96db at one meter, which most people will consider quite loud. I would suspect a lot of people don't really listen to speakers this small any louder than that.

The 30 watt amp will be able to drive the speakers 6db louder, all things being equal, and they probably aren't. 6db is pretty significant, but since the 6db is 6db over 96db, you may not notice it, depending on the music you're playing, the size of your room, and your average listening level.

Since these are single-ended amps you're talking about, they will have a sonic signature, and the transfer function differences between the amps just may overwhelm the power differences.

Have you ever connected your speakers (which are pretty good for speakers of that size) to a decent solid state amp of, say, 100w/ch, just for grins?