Back when you bought that one tubes were sort of like light bulbs... interchangeable and no big deal.
And, when you bought "a 12AX7" you probably simply got whatever brand they happened to have in stock.
(Or, at most, you paid an extra $1 for a "premium brand" like a Telefunken, which you hoped would last a little longer.)
Now tubes have become yet another audiophile cult thing.
Here's what an awful lot of people don't seem to realize....
Much like light bulbs, different brands of tubes do have slight variations in electrical characteristics (for the same tube number)...
These are what account for most of the differences people hear...
So, for example, a tube with slightly higher plate-to-grid capacitance may have a slightly more rolled-off high end in a certain circuit...
This means that, in a certain device, you may well hear differences between different brands and styles of tubes.
However, this does NOT necessarily mean that one is "better" than the other...
(Coke isn't better than Pepsi... and Pepsi isn't better than Coke... some people simply prefer one or the other.)
(And how would you feel if someone were to suggest that you should pay $100 for a glass of Coke?)
The problem is that some audiophile finds out that a certain brand of tube, with a certain color of plate, and a certain style of getter, sounds best TO HIM, IN HIS PARTICULAR PIECE OF GEAR.
And so he publishes a great review... or starts selling them for an absurd price in his "audiophile store".
At that point, audiophiles start INCORRECTLY assuming that the tube itself is somehow superior, and so will sound better in OTHER EQUIPMENT...
And, at that point, people start spending truly ridiculous amounts of money for a tube that's really nothing special at all.
In reality, unless you're buying it to use in the exact same brand and model of gear as the reviewer, then there is little reason to expect that tube to sound the same in YOUR gear.
(It is true that many modern tubes suffer from very poor manufacturing quality, and some are better than others, but this affects the life expectancy of power tubes far more than if affects the sound of peanut tubes.)
So, by all means, if you really convinced that a particular brand or style of tube will sound better in the particular gear you have, then go for it.
HOWEVER, if you're experimenting, you're far better off buying an assortment of low cost tubes rather than one or two ridiculously overpriced premium ones.
Odds are that, with several slightly different choices, you'll find one that you prefer in your particular piece of gear....
But, if you're imagining that a particular "premium tube", which happened to sound good in some other piece of equipment, will sound equally good in yours, then your odds aren't nearly as good.
(And you can buy a nice assortment of new and NOS 12AX7's on eBay for the price of a single "audiophile approved premium tube".)
The last time I bought a 12AX7 it ran me around 5 bucks....what happened?