rvpiano, I want to congratulate you on taking the next step to audionirvana. You are in for some silly happy surprises, big grins, and perhaps even some giggling like a little girl-- like the rest of us -- when we find the right tube for the job!
I’ve used tubes for 10 years or so now in various uses in my audio chain. and like you, questioned the tubes abilities, and the higher costs of "better" tubes.
If you just wanted to jump into tubes and get something going soon, I would suggest just calling perhaps 3-4 tube sellers to ask them what they would suggest based on your particular unit (but make sure to call Upscale Audio- they are knowledgable, will help guide you, will ask you what you are powering, etc. In fact, when you buy tubes from Upscale on their website, you are required to type in the type of tube unit that you would be using the tubes for-- in order to make sure they match electronically.
Sovteks and EH are .... okay. But the next level up you will hear differences in sound... say to the Gold Lions, as they said above. But the next level would be like modern made tubes that are well-designed and manufactured with good Quality Control (like PSVane, modern Mullards, etc), then like the NOS tubes that dont cost an arm and a leg, at the entry level rung, and on upward. I always try to buy the entry-mid level NOS that have really "low microphonics" means that the tube should have been tested on a tube-testing machine to check its current condition. eg, Upscale does this, like tube depot, etc will write down the "matched" electrical voltages/etc of each tube. so you know you got a "matched pair", eg, when I bought 2 tubes for a preAmp, and when I bought 8 KT88s.
But what type of tubes are you looking to upgrade right now?