High power amplifier and small speakers??

There is one consideration not mentioned so far. If you have a high gain amp (not necessarily high power) and a high gain pre-amp, coupled to high sensitivity speakers, you may have to operate the pre-amp at the very low end of its volume control, where there are more likely to be anomalies in channel balance, etc. Also if its a stepped volume control, the steps are typically larger at the low end and it may be difficult to get the "right" volume. Other than that, there is no problem with using high power amp with high sensitivity speakers.

Volvo: For about a year, I had the Parasound A-21 driving
the NHT Threes. They sounded fantastic together. The only reason why I got rid of the amp was because I upgraded
to huge monster Genesis speaks that were way too efficient......not a good match for the A-21, but beautiful with the NHTs.
Hope this helps.

As a general rule could one say that sound quality is diminished by using a powerful amplifier (150 w/ch into 8ohms) with small speakers (high sensitivity)?

No you can't generalize - this NOT a general rule. A powerful amp can be good. A low power amp can be terrible and vice versa.

You could say that a pure Class A amp is likely to sound better at very low ouput. In practice a good Class AB amp should work at low levels too. A Class B amp is likely to have higher odd harmonic distortion. So if you focus on topology then you can make some "generalizations".

In general you can say that small speakers don't play loud and don't have as good bass response as much larger speakers (this is box speaker physics). Small speakers generally image better (baffle physics) and are more appropriate for low level near field listening.