@jaytor,
In my experience the major determinate for output tube lifespan and reliability is the chosen operational points for the tube in the amplifier circuit and the heat dissipation level of the amplifier circuit. The more conservative these settings, the less stress/ on the tubes and thus longer lifespan.
I own the Coincident Frankenstein 300b SET mono blocks and I believe that the operational points are a bit toward higher/aggressive parameters. They sound exquisite but may be tougher on certain 300b tubes. I had an early (First production run) white base Takatsuki pair and one failed after about 2 years of heavy use(The other tested just fine).
Takatsuki replaced the tube without any hassle. They sent me the then new black based pair. One of these failed a few years later. So an amplifier with softer operational point setting may quite likely yield longer service from the tubes.
I've used EML XLS the past 6 years or so and they are a different story altogether. Ultra rugged, strong and rock solid reliable. They sound fantastic as well. Authoritative and dynamic sound yet organic leaning. Not as organic as Takatsuki but by no means clinical or analytical. They convey soul and emotion to my satisfaction.
I believe that no matter which premium tier 300b you select it will provide excellent sound quality. This level of 300b is admittedly expensive but are worth it in a high quality amplifier. No point in hampering this level of amplifier with lower cost lesser performance tubes.
Charles
