I have the YG Hailey 2 and I have been switching between three amplifiers, the Boulder 866, Pass INT-25 and AGD Duet class D. All three amps work well with the Hailey 2. The Pass is limited in power for louder listening levels but it worked well for my nearfield listening - it has been almost a year since I tried the Pass and I need to try it again. I might have a slight preference for the AGD Duets with better imaging and detail than the Boulder 866. I'm using a PBD MPD-8 variable volume control with the AGD Duets (no preamp), and using the PBD MPD-8 DAC at fixed volume with the Boulder 866 volume control.
I could probably be happy with any of these amps, but since I'm not using the Boulder 866 streamer and DAC, I have it listed for sale on Audiomart. The YG Haileys are currently at YG for upgrade to 3, so I'll start the comparison over again when I get them back and will also be comparing to the darTZeel preamp and 108 amp.