I am really happy with my Gryphon 120 - I’ve had (and have listened to) quite a few respected integrated amps, but I haven’t listened to the 300.
It sounds as good as it gets in my tiny listening room, hooked up to Harbeth Super HL5plus XD - I listen mainly to vinyl, and the phono module is also exceptional.
There is no used Diablo 120 for sale, otherwise I would have bought it. Can’t bring myself to buy a new one at full MSRP.
I owned the Harbeth Super HL5 Plus, not the current XD version. I can expect any Harbeth speakers to sound good with upfront and dynamic amps such as the Gryphon. I used to pair Naim with Harbeth, now it’s the Naim with Graham LS5/9.