1st,
Yea, i'm the BC from the Recording Website. Can't stand that place anymore. Sad to watch a good thing take a bad dive...
As for the EH tubes, you can check
www.8thstreet.com or
www.interstatemusic.com I think. I got mine through a local supplier. Going to the EH tubes in the Art units is a must do. They're like, $10 bucks a piece. Cheap and nice upgrade. And, it applies to any of the Art 12AX7 tubed systems.
As for the 2 units the poster has, what exactly is happening? It's cracking up, or dropping completely, or just a limiting type signal loss? Are you using the factory power supply on both?
Oh wait! I think I had this same problem once. I had a ground loop problem in my rack. Board was shocking me, etc... I would use the Art and everything would be fine, then signal drop, then screaming, etc...
I took it apart, removed the tube, cleaned the board and connections, replaced the tube, and re-assembled it. I then found my grounding issue, so I don't know if it was the cleaning or the ground problem.
If this is a possibility, Take the Art to another wall plug in your house. Check it there. (Extension cord optional.) See if your still getting the problem.
Blind Cowboy...