High-end amps almost always achieve their claimed wattage ratings, but there are amps and then there are amps, and in this regard, what Charles1Dad wrote is true: an amp with massive power supplies (and if it's a tube amp, one that also has the highest quality output transformers) will sound considerably more powerful than most amps of the same wattage rating. Very few amps fit this profile, however (and the ones that do are very expensive), so you're basically stuck with wattage ratings.
Amp's nominal power rating - any use?
I just paired a couple of Coincident Frankentein monos with my SF Guarneri Homage. The sound is great (fat, rich, dynamic, transparent) and sounds well with any type of music (opera, rock, electronic...). These are 8W monoblocks and sound like with more power or at least the same as my previous fabulous pass aleph 3 (30W class A SS). Of course it depends if tubes not tubes, class A or not, speaker sensitivity, impedance load, room dimensions etc, but what i see is that it's not a relevant criteria at all on its own. Maybe there should be some transformation formula to take into account some of these factors to get some Apparent or Perceived Power, but maybe it would be hard to take into account all factors. Any ideas, opinions, on this?