A few years ago I had a modest Unison Research Unico powering my Harbeth M30s (and before that my Spendor S3/5se). This made for some really sweet music without costing the earth.
After trying various ss separates I have reverted to an all tube integrated, the Graaf GM50, which drives my Verity's. This is by far the best integrated I have owned, and a keeper for me.
Anyway, there are lots of good choices for integrated amplifiers depending on your budget. You may wish to visit the Harbeth user group for suggestions (I think they also recommend Lavardin & Sudgen amps with Harbeths etc).
good luck!