More power for moderate listening levels?

I have heard, and liked, the HL5+, and to me, even with lower-powered tube amps, it can play quite loudly.  But, my idea of loud might be different from yours.  In building any speaker, compromises must be made, and I doubt that the HL5+ was designed to have extreme volume level as a strength.  The distortion you thought had been creeping in as the volume got louder might be something in the speaker and not necessarily a problem with the  amplifier you were using.  It would be instructive if you can find a more powerful amp to compare with the one you are using now.

I would personally be more inclined to be looking at amps that sound better at the volume levels I normally listen at, even if it means sacrificing some performance when playing at extremely high volume.  This is how I ended up with three tube amps, the one with the highest power rating being 6.5 watts/channel.

Interesting--lots of commentary here but apparently not by many that own or have experience with Harbeth's. I would suggest you check out the Harbeth user forum. Tons of information there posted by people using many different amps with YOUR speakers. As a former Harbeth owner (and lover) who ran five different amps with my Compact 7s before moving on to another brand and can say this: Harbeth's like--need--power. Each time I put more power in front of my C7's they responded with more jump factor, better dynamics, more projection into the room. You can certainly get good sound with 25 good watts, particularly at low volumes, but if you want to crank them up more power is better. That said, it sounds like you only want to crank them at the occasional dance party and who is listening critically in those instances? 

My 3 year old and 6 year old. 

Your comments are taken to heart. Many folks swear by the difference that power makes. My initial question which is perhaps somewhat naive is whether all that power acts in a way to control the drivers at medium to low listening levels. It’s just sitting there in reserve but can it affect sound quality. I think a few posters here suggest that it can.