OEM refers to the original (and factory replacement) equipment. NOS means "New Old Stock," which is the only way you'll get Mullard tubes. Or used, on eBay.
I was fortunate enough (when still practicing the engineering profession in broadcasting) to come across a cache of Mullard and RCA black plate 12AX7 tubes at a radio station I was working for over the weekend. I offered to clean out the transmitter building, and haul off the junk to the dump. The tubes were part of the deal.
Hey...they were about to back a pickup truck up to the back door and throw the whole pile of tube stock away. I just gave them a much better home than the landfill.
I called ART and they told me that if a better tube was going to make there units sound better, that they would of put the better tube in them. with that said, its hard to believe they would spend that kind of money by putting a telefunken tube or equivilant in those units.
I'm still researching. Does anyone know what tube type Avalon uses in their pieces?
Well...I don't give a rip what ART says. Anyone can hear the difference between the OEM 12AX7 and the Mullard 12AX7.
Of course, ART *could* be referring to the tube type, and not the brand. 12AT7 vs 12AX7 vs 12BY7. Or, they could be referring to tubes that they can get a regular supply of. You have to remember that these Mullard/Telefunken/RCA/Tungsol etc. tubes haven't been made in years. Finding a load of them in new condition is almost unheard of, and ART (or anyone else) couldn't reasonably expect to make a large production run with genuine NOS Mullard (or any other) tubes.
But...again...the Mullards (same type as the OEMs) greatly improved the sound.
I tried swaping with some Telefunkens and old RCA's. To me the difference was slight but it did improve. Remember most of the amp circuit isn't tube based so you are not changing the majority of the signal path.