AFAIK, what Bander says is exactly the case. Major problem is that most of the OEM tubes are not sold as matched pairs. But if your amp biases the tubes independently, that may not be much of a problem.
Western Electric 300b Tubes - Differences
So, sometimes I see these current production tubes for about $900/pair. Then I occasionally see some for around $400/pair. The less expensive ones come in the older style boxes. Was there a period where WE made crappy tubes? If so, when was that? How can you tell by looking at the tubes themselves.
Cheap:
http://cls.audiogon.com/cgi-bin/cls.pl?accstube&1150737895
Expensive:
http://cls.audiogon.com/cgi-bin/cls.pl?accstube&1149506422
Cheap:
http://cls.audiogon.com/cgi-bin/cls.pl?accstube&1150737895
Expensive:
http://cls.audiogon.com/cgi-bin/cls.pl?accstube&1149506422