Let's say you have two amps - one at 50 wpc and another at 5,000 wpc - with identical speakers and listening rooms. If you play both at a certain volume level, say 80 db SPL, both amps will provide the same power. However, during transient peaks, BOTH amps will clip. The 50w amp may distort at clipping while the 5,000w amp will recover much quicker where you would not notice the discontinuity. That's one rationale for mfr's pumping up the amp power - to tackle clipping.
So, the best way I can answer the questions is just how much quality you want for a particular listening level. You can get there with a 2 wpc SET as well as a 600w monoblock; "depends" is the answer - because a particular SPL level is dependent on room size, speaker sensitivity and physical comfort, etc. but the quality comes from both the electronics and the power capability. The SET uses its ability to produce a symmetrical waveform to recover from clipping while the SS beast uses brute force.
So, the best way I can answer the questions is just how much quality you want for a particular listening level. You can get there with a 2 wpc SET as well as a 600w monoblock; "depends" is the answer - because a particular SPL level is dependent on room size, speaker sensitivity and physical comfort, etc. but the quality comes from both the electronics and the power capability. The SET uses its ability to produce a symmetrical waveform to recover from clipping while the SS beast uses brute force.