I have three amplifiers: a 15wpc Pioneer 434; a 30wpc Technics SU-C04; and a 40wpc Technics SU-V3. I really can't tell them apart!
My 'Celestion 5' speakers have a
sensitivity of 88dB (1w/1m
Anechoic).
The SU-V3 has a peak power meter. At the moment it is peaking at 0.05wpc. When I turn it up really loud it peaks at 1wpc. In experiments I've had it up to 5wpc, but not for long.
My listening position is about 2m from the speakers. At 1wpc I have 88dB from the speakers, minus about 6dB due to the distance, plus 3dB because there are two of them, plus about 3dB to 6dB of room gain. So that's about 88dB to 91dB
SPL from 1wpc. That's already 3dB to 6dB over the 85dB that will cause
hearing damage with long-term exposure.
Also, I have noticed that my hearing quickly becomes less sensitive when listening to loud music. So I only listen to loud music for a few songs before turning the power back down.
I think I would be fine with a 15wpc amplifier. I'd double that for every 3dB below 88dB sensitivity and divide that be two for every 3dB above 88dB. And I'd quadruple that for every doubling of distance between the listening position and the speakers.
More important, to me, than wpc is
THD and the ability to sustain the required power level.