With respect to the original question, Nagra has been making bulletproof gear for a long time and I don't think the VPA is any exception...unless you mess with them.
I found the KR 845's to sound exceptional. Subjectively more powerful, with a darker tone and a richly saturated, more vivid sound. Very nice in my system anyways. Only problem is that one of the tubes "took out" one of my amps after some 20 hours. Said tube was said to test fine afterwards and last I heard Nagra was said to be working on a fix for this recognized problem with no resolution as yet.
I'm currently burning in the new 845M's and while they sound fine, the KR's sounded better. If I were a VPA owner, however, I'd probably stick to RCA spec'ed 845's rather than the KR's which are not as, for example, they use a different filament that draws more current. Therein may lie the problem. The VPA could probably be modified to use safely with the KR's.
One word about the new 845M's...one of my quad was misshapen such that I could not fit the tight VPA tube cage over it. This was replaced only after some hassle so when ordering make a point to mention your amp and these cages.
I found the KR 845's to sound exceptional. Subjectively more powerful, with a darker tone and a richly saturated, more vivid sound. Very nice in my system anyways. Only problem is that one of the tubes "took out" one of my amps after some 20 hours. Said tube was said to test fine afterwards and last I heard Nagra was said to be working on a fix for this recognized problem with no resolution as yet.
I'm currently burning in the new 845M's and while they sound fine, the KR's sounded better. If I were a VPA owner, however, I'd probably stick to RCA spec'ed 845's rather than the KR's which are not as, for example, they use a different filament that draws more current. Therein may lie the problem. The VPA could probably be modified to use safely with the KR's.
One word about the new 845M's...one of my quad was misshapen such that I could not fit the tight VPA tube cage over it. This was replaced only after some hassle so when ordering make a point to mention your amp and these cages.