Which Class D Amplifier? PS Audio, Ghent, Nord, Merrill or other???

Lot's of good options. Test them all and get back to us! ;)

Although I haven't personally heard any premium class D amplifiers, I believe that GaN will be a key technology and market driver.

For those of you unaware of GaN (or would rather just listen than geek out!), here's my analogy ... Non GaN transistors aren't as fast as GaN so it's kind of like listening to MP3's at a lower versus higher bit rate.

Personally I believe this is a great development for HEA.  Without getting into the differences between amplifier classes, GaN (at least on paper) should more accurately reproduce wave forms and require less filtering and other processing. With faster transistors we should get better sounding (hopefully great sounding) amps that are cheaper, lighter and more efficient.  Maybe then the ear bud generation will learn to appreciate great sound and expand the market.

To be clear I believe that linear amplification with the proper components will always be better.

Personally I'm anxious to watch the market over the next 18 months.  If the big guys (Yamaha, Marantz ...) start buying/using GaN in greater volumes prices will drop.  This would enable the brands frequently noted here to focus on improving the processing mechanisms for even better sound.

For what it's worth.

Bel Canto Design REF600M and the Black EX.

+1 on the Nord Acoustics, have the NC500 Monoblocks, I'm happy. You can also customize their sound slightly with interchangeable buffer boards and  op-amps. Mine have the Rev C. buffer boards, but I see there is a Rev D. buffer board recently released. It seems like an easy process to swop them out. I like that the amps are "user" upgradeable to a certain degree. Compact, light and runs warm to the touch even after some heavy listening.

Unless you've actually had Monkey Plague, you really can't say much about it.