Seekburk,
I own a Wright WLA12A preamp, which also uses 6SN7s. When my stock EHs got noisy, I wrote George for recommendations and got this response:
Dear Dave,
NOS are almost always a better choice for 6SN7's but the availability is getting harder to find these at a good price. My favorites are the mid 50's to late 60's production angled plate 6SN7GTA or GTB, RCA are also nice, the best are American made Raytheon, Westinghouse or Tungsol,good luck. The EH's are robust and fairly reliable as well, but a bit forward.
Best,
George
Taking his advice, I found some reasonably priced Sylvanias from the 60s, and they've been a major improvement, much smoother highs than the EHs. I tried some Sovteks and they were pretty bad in my amp, although many mfrs seem to use them as their OEM tube. I also tried some NOS GEs from the 70s that I got for free, and while they were slightly better than the EHs sonically, 2 out of the 4 were noisy. I haven't had the inclination to try any of the $100/pair 6SN7s, as I don't think my pretty basic preamp and system warrants them.
All in all, I'd rate the EHs pretty high considering their price and availability. You can improve on their sound with older tubes, but unless you're lucky enough to get them cheaply like I did, you'd probably have to spend a lot of money for the improvement.