I have heard both the ARC monoblock 450 wpc and stereo class D amps (225 wpc )with a REF5 , source was TT , Ayre CD player, AQ cables and Vandersteen 7 at HiFi buys ( at that time 2013 Audio Alternative)
other amps auditioned that day over a 9 hour period:
Aesthetix Atlas - Stereo
Ayre MXR
ARC Ref150
ARC HD220
while the ARC class D amps finished last ( I ended up buying an Ayre VXR, they put up a good account of themselves. Not the last word in refinement, smoothness, etc....they are relatively inexpensive and don’t require a long term care contract with a chiropractor
i suspect ARC and others will keep pushing the quality and refinement for obvious reasons.....as they should...just as the best more traditional circuit designers will also keep pushing, as they should.....