If you're really happy with the Harbeths, maybe you should consider keeping them and looking for a slightly more powerful tube amp. IMO, that'd be your simplest, surest and possibly cheapest solution.
Alternatively, of the recommendations made thus far, I like Klaus' Epiphany and the GMA Europa the best.
The Epiphany is amazing for it's size and cost, especially if you can pull it out into the room. The Europa is among the best small speakers I've heard, period. They do little wrong IMHO, and a lot really, really well. I'm not sure the 5.25 Ohm/88dB sensitivity is ideal for your application, but Roy claims they work with as little as 7 watts, so you should be alright.
Good luck with whatever you decide.