I was in the same situation a while back. Trying to find a pair of monitor speakers to match with 3.5 W/ch 2A3 speakers. Can't use horn speakers due to space consideration. The final solution was Coincident Triumphs UHS.
I am not sure what Triump's actual efficency measurement is. It was definitely loud enough for most Jazz trio and vocal performance, but not quite enough for pop/rock/symphony. The room size is about 18 x 26 x 9.
The final solution was to go to a 845 SET amp instead. 22W/ch. Very dynamic sound although not quite as transparent as 2A3 SET.
So given the choices again, I would say find a pair of used Klipsch horn speakers if you have the space. If not, I would suggest changing the amp might be a better option. Cary makes some higher powered 300B SETs.