I've read all the reviews of the HPA-1. They're all very positive, except Amir's. I've also listened to the HPA-1. It played loud without noticeable distortion. It was detailed but not really neutral, which makes sense given the design. It's a relaxing sound and gets a good groove on. That's just my subjective impression, don't freak out.
Reverse-engineered schematics for the HPA-1 are posted on diyaudio.com. I put the circuit in SPICE and it simulates almost exactly the same as the Stereophile curves. The HD650 is rated 100 dB/mW sensitivity. The HPA-1 shouldn't break a sweat driving these headphones to ear-splitting levels. I want to say, if Amir's listening test is valid, the review amp was probably damaged in shipping, beyond the loose control board connector. Or maybe it was just worn out from being left powered on continuously for years (bad idea).
I wish Amir had arranged to have the amp inspected at the factory before publishing such a scathing review.
Measured -83dB SINAD seems bad, but don't forget it's a log scale. THD of 0.007% is still very low, below the audible threshold, and it'll be lower at normal signal levels. There's no real need for amps with vanishingly low distortion. This is just a marketing gimmick, not an objective or "scientific" requirement.
I wouldn't buy this amplifier myself for a bunch of reasons. But I don't hate it for existing. I'm glad there are choices in the market. I appreciate that Nelson Pass employs American workers and provides them health insurance and benefits. You can build your own HPA-1 clone. Good luck making it the same quality for under a thousand dollars, and that doesn't account for the costs of labor, marketing, customer support, and keeping the factory lights on.
It legit makes me sad to see people hating on this amp, and I'm not even a Pass fanboi. I do get tired reading about cookie-cutter Chinese gear all the time. Pass Labs' design philosophy is different. It's a big world, full of injustice. There are plenty of more important things to lose sleep over. IMHO and YMMV.