Assuming you will be at your desk while listening to the Harbeths, you wouldn't need a ton of power. For a small footprint integrated amp, I would highly recommend the Rega Brio - it's half-size, 50wpc (8ohms), non-fatiguing sound.
If you intend to sometimes listen from a distance and want to fill a larger space, then more power would be required.
If you intend to sometimes listen from a distance and want to fill a larger space, then more power would be required.