Depends on your budget really. They're easy to drive so you don't need buckets of power, but they're revealing enough to present you with the difference between amplifiers, even far more expensive ones, and they'll respond well to more current. At the budget end, the Marantz PM6003, Yamaha AS500, Rega Brio and Rotel RA04se will all work well with them - which you prefer can only be determined in a demo. They really seem to come alive on the end of a Naim Nait 5i or Audiolab 8200A. If that's a little too far, the Arcam A18 will work well, and that can be had for around £470 at the moment. Other than that, there's not a great deal between £400-600 at the moment that's worth mentioning other than maybe the Rotel RA05se.
The best thing you can do is to get yourself a demo and try a few amps out and see what you prefer - there is no single type of sound that everyone likes, regardless of what some might say, and you're the one who has to live with the purchase, as well as pay for it.