Shaun,
A few things ....
The 7581 is similar to a 6L6, but in many amps it will cause problems due to screen resistors not being of a correct value. They are nothing at all like a KT-66, the curves are very different.
Current Svetlana's (made anytime in 2002, 2003, are low in vacuum, and are showing a very low service life, prone to premature failure, and the bias should be checked after only 10 hours of use. The GT version of this tube is the GT-6L6R2, and they are checked for low vacuum. If you buy Svet 6L6's make sure you know and trust your vendor. There is a popular internet tube vendor that has some pretty awful examples of these.
NOS tubes DO need to me matched. They were closer than today's tubes, but over the years of handling, I have found that in regard to power tubes, they are even more inconsistent than new made power tubes with less travel time on them. A lot of NOS folks with a limited supply, group tubes that are far from being matched into what they term as matched sets, as single tubes do not do a lot of folks much good, unless you own something like a THD Univalve or GT Solo Single type of amp.
Most any 6L6 is much better than the current production runs of the Svetlana 6L6. Hopefully this will change, as it seems to every now and then from that particular factory.[/QB]
Thanks Miles, That's very dissapointing about the Svet's. They have done me well in the past to last as long as they have with a fairly hot bias. I don't think I trust this vendor that much, he is ordering in some more 6L6's in for me and hopefully they will be in today as I want to retube and use the amp tonight. I'll see what he gets in.
Cheers,
Shaun.