I'm glad this thread came back to life... I was thinking about the HA-200, but then NuForce changed owners, et al., so I took a long excursion into tube-land.
Anyway, I am back to looking at the HA-200, based on the positive review commentary and the new, lower price (seems to be $299 on Amazon).
Most of the comments here are related to driving more difficult 'phones (e.g. HD-650, HE-6). For my HD-650s, I don't see needing to replace my Woo OTL.
I am curious how the HA-200 performs with lower impedance cans, like my Grado RS2i and AQ NightHawks, if anyone cares to comment.
Thanks, in advance.