I agree with the posters who indicate "it depends". My current Revel speakers are rated at 6 ohms with 87db sensitivity. They were initially being driven with a 50 WPC Arcam and I needed to push the amp pretty hard to get the speakers to sound their best. I switched to an 80 WPC Cambridge Audio amp and the speakers came alive. I noticed it especially in the lower frequencies. But there was also the matter of synergy. The CA amp, being a little more forward sounding, meshed better with the laid back sound of the Revels.
I had been using the Arcam with a pair of Paradigms that were rated at 92db at 8 ohms. That was another great synergy system. 50 watts was more than enough to make those sound great.
It depends.