There's a lot of mystique surrounding tubes. Especially the "NOS" relics that sell for up to $1,200 a piece! Now, why would someone pay that much for a piece of glass that could, without warning, die at any moment? Tone. Pure and simple. Sure, you could get a decent sound out of just about any tube. And, you could also ride a Vespa to work.
I used to collect tubes and try different ones to see which ones sounded better. I've spent hundred of hours recording the sound of my amps with different tubes and making A/B comparisons. Then, one day I realized that my hobby had grown out of control and my several thousand dollar collection of vintage tubes turned into a business. For anyone who wants to try "rolling" (the hobby of swapping tubes), here's a few thoughts from my experiences and suggestions to go along with them.
In a guitar amp, the input tube is usually the most sensitive stage and tends to have the biggest effect on tone so it's a good place to start. The input tube is most often a 12AX7 (aka: ECC83, 7025, and 12AX7A). In the 12AX7, you have a wide variety of tubes going all the way back to 1948. The earliest of these tubes are the American "black plates", nicknamed such for their appearance. They have a deep, smooth, and rich sound. After the black plates went away in he late 50s, we still had some decent American tubes with long, gray plates. These are also fine sounding, with a little more character and an upper midrange peak. In the 60's the quality started to drop off a little and we were left with shrot, gray-plate 12AX7s of varying quality. They are more mid-range focused and aggressive. The prices on these are still reasonable and they beat the hell out of any tube being produced today.
The chaps in England made the famous Mullard tubes which are reknown by guitarists for their smooth, rich sound. Nothing can tame distortion "fizz" and harshness like a Mullard. The common types are the 60's short plates but the rare 50's long plates are especially nice.
In Holland, they like a very musical and lively tube. In comes the Amperex. Sounding good in almost any situation, the Amperex impart an exciting sound while still remaining somewhat transparent. The bass has a lot of groove and the highs shimmer. If you can find one with the "Bugle Boy" cartoon on the glass, hold onto it!
The Germans are known for paying great attention to detail and it is reflected in the tubes produced there. Telefunken, Siemens, and Valvo are some of the most accurate and detailed tubes ever made. For a clear, "airy" sound, they are unsurpassed. However, when pushed hard, they will not go into a smooth overdrive like the tubes previously mentioned.
My choices vary depending on the mood and the tone I'm after. In my blackface Fender, it's usually RCA (the original tubes used in vintage Fender amps). In my Engl Fireblall, I use a mixture of Telefunken, Mullard, Amperex, and RCA to reach tonal nirvana. I highly recommend getting a few used, vintage tubes and trying it for yourself. Most of the types mentioned above can be had for $15 to $50 each, used if you look around. But, it's addictive. So don't say I didn't warn you...