Issues with KR 845 tubes in Nagra VPA Amps?

Have other Nagra VPA owners experienced problems with KR 845 output tubes? A couple of months ago a KR 845 tube in my Nagra VPA's failed. My local dealer no longer stocked Nagra products so I contacted John Quick of Tempo Sales. John is the national distribution representative for Nagra high end products and Verity Audio loudspeakers. John told me that the KR tubes were subject to shorting and creating collateral damage in the VPA's when doing so. John said that Nagra had created a "protection circuit" to prevent the collateral damage.

I arranged with John to have the protection circuits shipped to Steve Huntley of Great Northern Sound. For a host of reasons, I was reluctant to ship my VPA's and John was good enough to accommodate my request. (He was very helpful and responsive.)

Steve Huntley did a great job of fixing the VPA's and, so far as I can tell, the amps have never sounded better. I do not think that the protection circuits had any detrimental effect.

Have other VPA owners had problems with KR tubes and gotten the protection circuits?
A friend of mine expirienced a tube failure twice in his VPAs. There was no other damage other than the tube itself. Each time tubes were replaced by Nagra under their 2-year warranty free of charge.
I have a pair of VPA and PLP....with changing the original tubes to KR845,,, it is working fine for almost 2 years... The failure may be caused without TUNING the load meter properly , there is 2 small hole at the right hand side of the VPA side pannel ..the user package also provide the screw driver and the spare fuses....please refer to the user manual for how to tune the load meter .........Remember to mark down the original setting before starting the tuning....Just in case , if you what to use the original 845 tubes .... Hope it may help.