TL;DR: It sounds pretty nice, but for the price, I would not be satisfied. Maybe that’s the point though? Half kidding. Mostly kidding. : )
I can’t compare it to the HA-300, but I compared to the Fidelice (Rupert Neve Designs) Percision DAC + Headphone Amp (~$5,000?), and the Fidelice unit wins for me. Ok maybe not a fair comparison, but even the Fidelice Percision Headphone amp (~$1,000) seemed more satifying, enjoyable and memorable.
Not to say the Cayin sounds bad. It sounds pretty nice. The thing is, for that price...it should sound better if it’s sole function is to be a headphone amplifier.
Full disclosure, I’m not a fan at all of using certain newly produced tubes in the signal path like they did with the EH 12AU7’s they put in there (which IMO vary from not great to downright bad usually). So maybe that was partially why.
But, again...it’s a $2500 headphone amplifier...this is endgame pricing territory; it shouldn’t use those tubes in it haha. Maybe it’s better suited for other headphones than what I used, and I don’t remember what they had hooked up to it, but I think it might have been straight from my Cayin N6ii possibly - long day there hah.
Edit - more that I think about it, more I was slightly disappointed. I had heard their other amplifier was very good, so I was excited to try the new model. But I dunno man. I have ultra linear tube monoblocks from the 50’s sitting right next to me, connected to my home stereo. The Cayin didn’t live up to what I’ve come to know and appreciate from the ultra linear/Williamson circuit design. I’m sure it will have its following though.