@jo1mtb, I have no experience with the Boulder 866 integrated amp. My retailer carries the Boulder 866 amp but strongly ‘recommended’ the Ayre AX-5 Twenty amp for its excellent build and sound quality. I eventually sold my SimAudio 700i V2 integrated amp and purchased the Ayre AX-5 Twenty integrated amp.
My Ayre AX-5 Twenty amplifier sound is different in that the sound is more natural and less 'exaggerated' than my previous amps. This is very hard to put into words. The sound is there with no artificial extra stuff around it. It seems my previous SimAudio 700i V2 integrated amp exaggerated the music, and this Ayre amp does not. The music sounds like music with no extra stuff in the way if that makes sense.
I hope this helps. Thanks....