I don't have experience with that amp, though I do have a similar 2-tube version using the 6N3's.
I plan on replacing mine with GE 5670's, preferably the red star version based on reading what some people I trust have had to say. I've used the white star GE5670's in previous pre's and I liked those a lot.
I just got my 6N3 put together a little more than a week ago, so I'm planning to use it as is for now and just see what I think.
So far, I'm very, very happy with it. Blown away, actually.