Wilson Wat Puppy 7 or Avalon Opus Ceramique?

The Wilson is easier to drive and the tweeters are more polite than the Opus Ceramique's. That's based on my experience having owned Eidolons for 11+ years and listened to both Opus Ceramique and Wilson WP7s. WP7s do not sound like WP5s and WP6s. They are actually extremely buttoned down and can work well in small rooms.
The Opus Ceramique will not give you any possible bass problems with woofers overwhelming the room but don't expect the highs to be as detailed and refined as the WP7.

And then, of course, there is the Fletcher-Munson curve to consider, if Mike is going to play music at low volumes. Given that, a small, high-quality monitor with a sub (or subs) might be a good choice. I suppose he could get an integrated amp with tone controls instead, but that wouldn't be my first choice.

Mikefi: what does "rather low volume" mean?

The times I've owned monitor speakers I tended to listen to them a little too loud in order to get the fullness in the low frequencies that I enjoy. When I put a sub with them I enjoyed them at a lower volume, relying on the sub to give a subtle fullness to the sound. I have since moved to larger speakers and don't use the sub anymore. I have a fairly large listening room, so this thought process may not work for the original poster, but it's just something maybe to consider.

For a monitor that sounds amazing at low to medium volume, check out the Harbeth Compact 7-CES. They sound amazing and are not that fussy about electronics.