I think I have the 209, but since the NT-6 Pro is my main earphone of choice, I lost my motivation to continue rolling op-amps in the UHA-6S after powering up with the OPA627.
I personally think the 627 is the best possible match for the Hidition, they complement each other so well.
However, the 627 makes the Heir 4.A sound dead -- spacious, but without dynamics or treble. It's unappealing when compared to using the 8610 with the 4.A.
It's an excellent demonstration of how essential equipment matching can be. If only I wasn't using custom earphones, I could show others how much this matters, too.