I have been running Duo’s since 2006 with Atma-Sphere S30 Mk3 OTL stereo amp. The amp is dead quiet and produces a wonderful sound. At ear-splitting levels it wasn’t even showing 1W on the meter.
I’ve recently changed to AGD Audions and these are also a terrific match with the Avantgardes. You could also have a look at Atma-Sphere Class D monoblocks which are getting great feedback.
I agree with @lubachi that you don’t necessarily need to spend a lot on an amp for these speakers as they are so easy to drive with their high impedance/efficiency.