Did you test the 6KG6? It's odd that one is totally dead, unless something caused it to blow. Does the set use a damper tube? I'd check that as well. Here's the link of who I order tubes from. I see he has "call" by the tube you want tho. That means he either has little to none in stock, or it's pricey
http://vacuumtubesinc.com/index.html