I've not heard the NAIM so can't compare. What was a revelation is that I have some headphones that are high-end, but to whom I've never really warmed up to. I think the old sense that you should love XXX due to the price, reputation, etc. But I could never put my finger on the cause. I kept thinking maybe it was the player, the DAC, the whatever.
Using the HA-1 put all those thoughts into absolute clarity. it really showed exactly what was wrong with the headphones, and conversely what sounded really good. With all consideration, the HA-1 driving the PM-1 is a joy.
I was actually thinking that the PM-1's wouldn't sound better with the HA-1, especially in light of their extraordinary sensitivity. I used the Cambridge DAC Magic, the Korg, and the AudioEngine D3 before the HA-1 arrived, so I felt I had a sense of them, and once the HA-1 arrived, it was a different experience. Without commenting on the DACs, I can say that they HA-1 provided a richer, fuller sense of the music. Not only did the PM-1s sound better, so did everything else. Except for the headphones I wasn't thrilled with, which it helped pinpoint the issues.
I need more time before I can make sense of this, but I think that Oppo has a hit on their hands. It's not cheap, but for a pure class A system, with top flight DACs, and lots of flexibility in terms of input and output options, it is fairly priced, and completely solid.
What are some of your reference points that you are comparing the Oppo too? Which amps/dacs w/ which heaphone(s)?