Hi, well, you are starting on a fun, but somewhat time consuming and potentially expensive journey.
At the end of the day, tubes can make a difference, but those differences will be somewhat subtle; those subtleties will be more magnified if you have very good equipment (the amp, headphones/speakers, source, etc.) and less pronounced otherwise.
Tubes can reveal more or less defnition and they can be warmer or cooler; some of this will be a matter of taste and some will be a matter of which qualities create synergy with your system.
When picking tubes you can go for brand new tubes, older but unused tubes (sometimes called New Old Stock or "NOS"), or used tubes. Tubes have useful lives after which they are no longer going to work well or sound good. In order to know how much life is left on a tube you need tube measurements. There are a number of tests that you can run on a tube (if you have a tube tester), but the actual numbers only means something if you know what tube tester was used and what the normal test results are for that tube on that tube tester. Good tube sellers will publish the test results, the normal "new" or "good" numbers for that test on the tube, what tube tester was used, and possibly other test results (such as whether the tube tested good for shorts, gas, etc.)
Tube testing isn't rocket science, but there are good honest human testers and good calibrated machine testers. If you buy tubes on eBay or Audiogon, generally (but not always) the seller's information in their ad and their feedback ratings are indicative of their skill and honesty.
One thing to keep in mind is that some tubes not only need to test well, but they should have comparable (within 5% or so) test results per pair as they operate as a matched pair in an amplifier circuit. (Some hifi amps have matched quads or matched octets). Also, some tubes (such as your 12AX7s) are triodes that have two sections within each tube that should be matched (the two halves should match) - so again, you need the test results to know what you are getting.
Beyond testing is a lot of opinion; some people including many with experience are of the opinion that older tubes that are still in in good shape (expecially NOS) will sound better than newer tubes. This thinking is reflected in prices as 1940s, 50s, and 60s tubes generally cost more than 1970s and 80s tubes. This thinking says that materials and designs were optimized for performance during these periods (and perhaps work ethics were better) vs more modern products that were optimized for cost-reduction. So you will get people saying that older D-getters sound better than newer O-getters, and so on and so forth; others will say the design didn't matter but the older tubes sounded better just because they were made back in the golden age of tubes.
Among tubes, Tong Sol, Telefunken, Amperex, Mullard, GE, Philips, Sylvania, Raytheon, and many others made tubes that have loyal followers. Today, relatively few tube manufacturers exist; Sovtek, Svetlana, and Electro Harmonix are popular brands. Tong Sol also has it's name on some new tubes but there is some debate about whether the brand is synonymous with the old quality.
The only way to know for sure about a tube is to test it and listen to it. Before you go down that path you can get lots of opinions about tubes on the vacuum tube forum at
www.audioasylum.com and other hifi and tube web sites.
Happy tube rolling (that's the term for trying tubes.)
Yada
