I can't give you a direct comparison because 1) I don't have a Boulder 866 here and 2) the biggest factor in any stereo system is the room.
What I can tell you is that I have a very nice system with a First Sound Presence Deluxe fully dual-mono preamp and very high-quality (formerly TOTL) Conrad-Johnson stereo tube amp, that, in my room and with the rest of my system is very, very comparable in audio quality to their ART150 stereo amp, and....a Constellation Inspiration integrated here, and...I frickin' LOVE the Constellation Inspiration integrated. Literally listening to it as I write this. The only other two integrated amps I've personally heard that can compare to it are the Constellation Argo and the DarTZeel CTH-8550.
So, I can't tell you how it compares to the Boulder. What I can say is I could have the Constellation Inspiration Integrated amp as the "centerpiece" of my system and live with it forever and be very happy, indeed. I think it's....fabulous.