Not enough power what does it sound like?

It sounds like s... !

It took a lot of power to drive the bottom end of my known-to-be-hard-to-drive B&W N801's.  That was the biggest difference I can say was for-sure related to power output as I experimented with a Crown XLS-2502 which has tons of power.  It had other issues.  Switching ("permanently") to my Levinson 333 made all of the difference.

Just to agree with the above, it is the underpowered amplifier that blows the speakers. The clipping/distortion makes the speaker move in ways that it was not designed. Uninformed people think that it is too much power that blows the speaker. It is really the underpowered amplifier that clips and blows things us.
Remember that there are muscles in your ear that tighten up when exposed to loud volumes and decrease the movement of your eardrums and protect the delicate hair cells that would otherwise be damaged by high volumes.. That is why when you walk into a room the volume seems louder than it does a few moments later.

+1 @shadorne

Muisc always sounds louder with an underpowered amp. Folks are sometimes tricked into thinking their underpowered SET tube amp has enough juice to drive their 88-92 dB speakers because it plays them loud. Well it may be a good match or it may well be the added noise from distortion.

Not enough power = clipping.  

https://en.m.wikipedia.org/wiki/Clipping_(audio)

Avoid it at all costs.