I appreciate the feedback on my earlier post. I ended up sampling the HA-1 with my HE-6 and TH-900 at Oppo's office in Menlo Park (as well as their BD-105, PM-1, and PM-3). They graciously allowed me to sample all of these products and were curious as to what I thought of them. I particularly enjoyed the PM-3 with my DX90. This would be a fantastic mobile rig. They were also really cool about me being there blasting music through my headphones on a normal work day! I felt bad since they were all working and must have been distracted by my music but they did not seem to be bothered.
Since I highly enjoyed it, I decided to purchase a refurbished HA-1 (it looks and feels brand new, and is packaged as such). I don't think one could distinguish it from a brand new unit. I love the interface and customization of the screen and wish this type of interface was used on more products.
I brought it home to my home rig and paired it with my LD VI+ and HE-6 and found the sound to be a bit too bright. Pairing with a warmer amp could help. Coming from a DacMagic as my previous DAC, I heard details in the music that I never knew were there. The HA-1 is very revealing. I also used the TH-900 (balanced) and TH-X00 through the headphone amp of the HA-1. I found the TH-900 slightly bright and analytical as well, but thought the TH-X00 with its smoother treble was an excellent pairing. I sold my HD-650 years ago but I think it would also be a fantastic pairing with the HA-1.
I'm no authority on the subject, but my conclusion is that pairing the HA-1 with a darker headphone is ideal. But when pairing with headphones a bit brighter than neutral, be prepared for a little extra treble sizzle. It was a bit too much for me. Since most of my cans are a detail oriented and a little treble happy, I have decided I will be returning my unit and will start to look for a DA8 or DA8 II. If all or most of my headphones were darker sounding, I would surely be keeping this unit.