Well I went back and redid the gain setup on the pa2v2 using my hd 555. No success.
I'm now convinced the pa2v2 is simply not good enough for a pair of headphones of 55 ohm impedance and up. You lose huge amounts of sound definition and quality. The sound is simply gutted.
I've found the max impedance for the pa2v2 is a 32 ohm headphone after testing it with all my gear.
I'm getting the impression lots of people are completely oblivious to the true potential of their high-quality headphones.