To my middle-aged ears, there is a noticeable difference, meaning improvement over current production Mesa preamp tubes. I play mostly low to mid-gain stuff, except for the occasional Mark IV creamy solo. Vintage Tung-Sols preamps in V1 and V3 of my Mark IV increased the vintage vibe, eliminating most of the "Modern" voicing I don't prefer. I run Amperex ECC83 in V1 and V3 of my C+, and a Tung Sol in V2. The Amperex is a little brighter than the Tung Sols, which I prefer in the C+ (with bright reduction mod). Again, I don't play metal, so for me vintage tubes are great.
With power tubes, I've had luck with new production STR 420s and 5881s in my Mark IV. But the jackpot was scoring two vintage pair of Sylvania dual getter 6L6s for the C+. These were also labelled Mesa STR 415. I was skeptical at first, but there is certainly an improvement over the current production tubes I'd been running in the C+. I'd rolled between 6L6 Winged C's (also great), STR 440s and Groove Tube 6L6 GE. But the two pair of Sylvanias are it for me...not surprising since these were the tubes in production when the C+ was conceived. REMEMBER: when considering power tubes, they must fall within Mesa's fixed bias operating specs. Reputable dealers of new tubes will know which tubes fall into this range.
Concerning price, you can get good deals if you're patient and know what to look for. There are good deals and good dealers on eBay; there are some losers there too. You'll need to understand what the various tube tester measurements mean (mutual conductance, emissions, current draw etc..). Or you can buy from on-line dealers, but they get pricey. I've gotten great preamp tubes for less than $30 shipped, even less if the auction is for more than one tube. It helps to buy from someone who accepts returns as vintage tubes can test great and be noisey or microphonic. Patience is key. Try one in V1 and go from there.
