These Mesa Boogies 12 AX7 valves represent a good middle ground between the cheaper budget end of the pre amp valve market, and the more expensive end.
The very cheapest pre amp valves can have issues with their consistency and reliability, and are best avoided. At the opposite end, you can pay huge sums for these valves. However, I would challenge anyone to truly hear the difference between the Mesa Boogies and the really high end units. Remember, there will always be people that will pay ridiculous money for what they think are the best - and the manufacturers know this set prices accordingly.
Be sensible and buy these - not unreliable and cheap, but not over-priced and with the reassurance that the Mesa Boogie brand brings.