This'll really throw a curve at you. My main amp is an SET 300B, 9 watts of SET bliss.
For a back-up amp I built a CDA 254 amp from Class D Audio, 250 Wpc.
This low-cost Class D amp has blown me out of the water, as it compares very favorably with the sound of my SET 300B (which, by the way, I'm using EML 300B XLS for output tubes), but with more power.
At a little over $400 for the complete kit (which are USA made, and very good quality), I recommend them for driving your full-range electrostats. There has been a lot of positive comments regarding Class D here lately, and there is good reason for it.
And the usual disclaimer, I'm not in any way affiliated with Class D Audio. But I did just order another CDA 254 to run in bridged mode, so I can run the pair as 500 Wpc monoblocs.