How do the Electro Harmonix 6sn7's sound?

It is not a bad sounding tube at all, pretty linear and fast. Sounds about as good as a Syl GTB which means you can do a lot worse.

EHs in general are a little edgy and can be slightly analytical in the highs. I noticed this consistently when tube rolling my CJ MV55, whether it was the EL34s, 6SN7s or 12AX7s. But they aren't bad tubes at all and in a soft system, they may be just right.

Arthur

For an inexpensive new 6SN7, I obtained superior results (in the same amp Arthur mentions above) with the Chinese "Ruby" version (Shuguang factory I believe) than the Russian Sovtek (Reflektor factory, same as EH, though I don't know if or how they might differ beyond the pins) -- and I haven't often liked Chinese tubes.

I have a Wright WLA12A, and find the EH 6SN7s very listenable and a bargain at $8.95/ea. They're far superior to the Sovteks I own, both the 6SN7s and the supposed "milspec" 6H8Cs. If all NOS tubes went away and I could only get new production tubes, I could easily live with EH.

That said, there are definitely NOS tubes that sound better. I have Sylvania GTBs that are my current favorite, and GE GTBs with the top filament that are a close second. I didn't have to pay a lot for either, and I've never tried any really expensive tubes - not sure my preamp or the rest of my system warrant them.

I'm interested to try the Shuguangs...with the metal bases, you can dream that they're $400 Sylvania Ws...

:-)

Jppenn, Do yourself a favor, you don't have a huge tube roll here. Don't mess with the non NOS tubes. It's a waste of time and money. Go for the old valves. Experiment a little. For me all of the Sylvania's the W(40's), GT & GTA(50's), and GTB(50's to 60's) are wonderful and are far superior to the current manufactured tubes such as the Electro Harmonix you mentioned. Sylvania has it all sonically, few could argue. The Sylvania 6SN7 IMHO is the finest American tube ever made, bar none. In general they are progressively better the further back you go by some measure but that said the GTB's have smoked any current production tube I've ever heard in my applications. Just keep in mind that the price goes up with the scarcity and age of the tube. I run ten of them in my Atma-Sphere OTL MA-1 amps and have never heard these amps sound better. I have a friend that runs them in his phono amp and loves them.
Happy Listening!