I have a Peavey JSX 120w head that I'm changing tubes (valves) on. I have 4 EL34 power tubes and 4 12AX7 pre amps. From stuff I'm reading, apparently you have to match your V4 or phase inverter tube with your power tubes? Is that correct?
Tune low and Play slow

Not even sure what that would mean, there's no "matching" to be done there. They're totally different types of tubes used for different parts of the circuit. There's nothing to match between them in any meaningful way. I've never seen such a claim. What exactly are they saying? You need the same brand? This sounds like some greasy salesmanship.

Power tubes must be matched among themselves (so you need 4 matched EL34s) and some people use balanced PI tubes (not at all necessary, but not an uncommon practice) but there is no matching between the PI and power tubes.
Roc8995 Ohhh ok! It's balanced they were talking about. Ok so what do you mean by that then? Could you help me understand as to why I should have a balanced PI?
Tune low and Play slow
The PI splits the signal into two halves so that the power tubes can amplify it more efficiently. Basically half the tubes "push" and half "pull" and the phase inverter is the thing that splits the signal into "push" and "pull" halves. The balanced PI use in tube guitar amps is a legacy from other applications where it was genuinely important, but it's not beneficial for guitarists in most cases.

In hi-fi world, where you want a pristine squeaky clean signal, it's pretty important to balance the phase inverter circuit to make the halves exactly identical (except inverted, of course). So you use a balanced tube for that. A balanced tube works like this: a 12AX7 for example is a twin triode which for our purposes means it's two amplifying units (triodes) in one glass envelope. Each of those triodes feeds half of the signal, so to balance the PI perfectly you sort through and measure tubes until you find one with triodes that amplify exactly the same.

In a guitar amp, though, we don't care about fidelity, and in fact we mostly don't like it at all. It doesn't sound very good. So a lot of amp circuits actually imbalance the PI on purpose, so that even if you put a perfectly balanced tube in there, the circuit is going to throw it out of whack because whoever designed the amp thought it sounded better that way. So you can see immediately why it might be a bit silly to insist on a balanced tube in the phase inverter position. Now, it's entirely possible that a non-balanced tube might be so unbalanced that it puts the PI back into balance, or might push it so far out of balance that it sounds really bad. That's rare, but possible, so it could be one reason to use a balanced PI tube. They're not usually much more expensive so if you're really curious there's no harm in trying one, but don't expect to hear any real difference, unless the old tube was shot, wildly imbalanced, or a different type or whatever. Even amps that don't unbalance the PI will usually have significant component variation or drift, or generally are not sensitive to how balanced the PI is. Some amps may benefit from a balanced PI, but in the vast majority of guitar amps, it's not going to make any audible difference to have a balanced or unbalanced tube.

I think a lot of tube dealers continue to suggest balanced PI tubes because customers don't care and won't ask questions about the $2 extra or whatever it is for a balanced tube. As a consumer you don't feel cheated (it's only $2!) and maybe you feel a bit more secure that you're being extra careful or "just in case" it makes a difference or whatever, and you don't care because it's not that expensive. But the guy selling the tubes is more than happy to take $2 extra every time someone buys a PI tube, and multiplied over thousands of orders that really adds up. Plus he'd rather just sell everyone a balanced PI tube than try to explain that one in a hundred situations where it matters. So you'll keep reading about needing a balanced PI, because it's not really hurting anyone in a major way but people who sell tubes are happy to make a little extra money every year from folks who don't care enough to wonder if it's really necessary.
Roc8995 Wow, you sir are a wealth of knowledge. Thank you very much for your help. I'm now going to shove that PI tube in and play for a couple hours haha
Tune low and Play slow