Cool. My suggestion of using a 30-second sweep and slower pot rotation still stands.
Tom are you using a Linear pot?
I a/b HPA-1 vs Monoprice THX 887 today Single-Ended, it no contest HPA-1 is much nice AMP working a broad range of headphones Dan Clark ( Mr. Speakers) Aeon Flow Open, Hifiman Ananda (25 Ohms), Rosson Audio RAD-0 (29 Ohms), Audeze LCD-4 ( 200 Ohms). I had zero problems driving any of these headphones with HPA-1 without adding gain. THX no gain is level 1, I found I need Level 2 gain with THX with LCD-4 to volume match at a moderate level of volume.
With Rosson Audio RAD-0 I had 1/4 Single-Ended to HPA-1, and XLR out to Headphone from Monoprice THX 887 4 pin xlr jack, I still preferred the output of the HPA-1 Single-Ended. You have plenty of volumes, a very clean signal, and great dynamics.
Loving the AMP more and more