Speakers with Cary 300 SEI

I urge you to listen/read, to fullrange, single driver, crossover-less speakers. This is where the "Magic" happenes with the 300B, zero-feedback SET designs. Since you are looking for monitors you can take a look at Omega speakers. They are very efficient as well. Also if you don't mind floorstanders Cain & Cain also fits your bill.
Good luck!

Lex- Omegas are made in Norwalk, CT and the owner is reputed to be a real nice guy; he was pleasant and prompt in a single email exchange he and I had.

I second a listening of the Harbeths. Their upper range speakers are both bulletproof and a nice listen. If you are considering the Proac's; listen to the Harbeths. I think they're a better speaker for a number of reasons. I would also try a couple of unorthodox things like Joseph Audio's fourth order crossover speaker. Especially the RM7. Or maybe something that might seem wierd like a Martin Logan. Or Green Mountain. In that you already have a crossoverless though (the headphones will be a no crossover design) you may want to consider expanding your horizons, and experiment.