Tube kit for Nagra PL-L?


Nagra recommends that users purchase a "tube kit" from them as tube replacement. This at a price of a couple hundreds of $. What is so special about these tubes? Aren't these Sovtek tubes that are easily available off the shelves?

Any one made any comparison with these tubes?
shsohis
I've had a tube get noisy on my PL-L, I've order the tube kit b/c of the comments above on testing/specs/matching Nagra does.

On the side, I have tried a few other 12ax7's (The TJ Full Music 12ax7's are pretty nice-more detail/bottom end...brings the image forward somewhat). You do get a gain here or there, but seem to loose something here or there also. The stock tubes seem to be just about perfectly balanced on all parameters.

I think the Nagra tube kit is about $250. So if you buy a few 12ax7's doesn't take long to burn through $250 in NOS tubes...and be underwater in comparison...

Let us know what you decide...
@Mcintech: Are the tubes the same in either kit? You just don't get all the ancillary "bling", in which I have no interest anyway.
Exact same tubes without the accessories. Contact Nancy at Nagra USA; (615) 451-4168. She'll have to order them, I think.
Jfreck - I've tried and permutated with most of the acclaimed tubes out there and yes you win some but lose some.... After one big big cycle, I landed back to the stock tubes.... Quite a joke right?