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?
Hi, not sure it's a joke...just tells me the stock tubes are very well matched to the the circuit in the Nagra...

The jokes on us maybe for spending more money on other tubes :-)
I am sure there are many people who would be happy with the stock tubes which a manufacturer selects for quality out of larger lots. However the tubes the manufacturer must choose from are only those that are currently available in large enough numbers to permit a reliable supply chain. The sound of those tubes are still limited by the inherent quality of the materials and processes that go into making them.
Many old tubes are indeed noisy or have other faults but there are still quite a few around that simply sound better than the stock tube does no matter how carefully selected. I love my old tubes and yes, I got some that aren't good, but I got many that simply blow current production tubes out of the water.
I must say that I am thankful that great strides are being made in current production. I have been very impressed with the current Gold Lion tubes in particular. Indeed some manufacturers are using the GL tubes, but I remain amazed by how many products come with the cheapest tubes around. I wonder if there isn't some silly misguided penny pinching going on in some cases.
How do you know if your tubes need to be changed? How many hours?I looked inside my PL-L because is thought there was a usage gauge but I did not see one. I just googled and I do not think that this was in my model but apparently was in older models.

It could be my imagination but I do think the sound is degrading. Not as liquid and maybe a little harsh at times. I do not have a lot of experience with tubes so curious of thoughts.
Do yourself a a real favor and get your selected very low noise and well matched tubes from Jim McShane over at audio asylum. Very fair prices, great service and advice. I have bought mainly new production but also a small number of NOS My experiences with him have been really positive.
P.S. I am not a shill, just very pleased with this dealer.