Anyone compared 6922 Sovtek & Electro-Harmonix

I can advise of both in a Quicksilver Pre-amp. I find the EH much smoother and more lush. As Newbee mentioned I too find them a bit muddy in the bass department.
Sovteks have better bass but can be a bit edgy with vocals and symbals with my pre-amp.
Sorry I do not mean to digress from your question on ARC.
Go to Upscale Audio's website and search for 6922/6DJ8 - you will see some reviews of the EH listed as type 2 I believe, there is a review from a user of the EH tube with respect to his LS-16.

I've used both in my BAT VK-P10 phonostage. Switched to the JJ gold pins. More money but worth every penny. I agree with the above posters on the EH vs. sovtek. EH smoother but slow, Sovtek a little to edgy for my tastes. The JJs have proved to be the most reliable of the three also.

I tried the E-H 6922 gold pin in an ARC LS16 and didn't care for it; kind of bland. Liked the Sovtek 6922 much better.

I am presently using JJ E88C/6922's in my Audio Research preamp and find them to be very good. Of course my first choice would have been Amperex or Siemens, but I just couldn't afford to spend over $1,000 for 8 tubes.

http://thetubestore.com/teslaecc88.html

ARC LS-7 here. Agreed. Thought it was just a burn-in issue. However, it does appear that the EH tubes are notably more polite than the Sovteks.