Not your everyday bookshelf speaker?

http://www.stereophile.com/standloudspeakers/1205lipinski/

A bit of clarification re: my post above.

I think folks have identified some very good monitors here (specifically, I really like the Lipinskis), but....

IME, the amplified Sonos Zone players always sound a bit tinny, regardless of how little they are stressed by a speaker's load. More sensitive speakers aren't IMHO the answer here. Worse yet, better (more revealing) speakers are simply likely to make the internal amp's shortcomings more obvious. That's why I suggest bypassing the amp entirely (or going for the unamplified zone players).

A powered ZP is fine for casual listening, but if you're looking for something that allows real scrutiny, I'd go for an external amp or powered speakers.

If your budget allows, powered speakers from PMC or ATC are excellent choices, as well.

Marty

Bookshelf and efficient with any kind of bass is somewhat mutually exclusive. The NHT Classic 3 is an excellent bookshelf speaker, but only about 89 db efficient, which isnt bad but it sounds like you are looking for more. So why not try powered speakers like AudioEngine. You do have to have an outlet nearby, but self amplified speakers can resolve frequency response issues that are difficult for regular bookshelf speakers to overcome.

I like my Ushers X 718

wilson benesch vertex or trinity. efficient, though maybe not highly efficient.