First Tube Amp: Advice? Keep spare tubes on hand?

Is there a priority on what spare tubes to get first or to keep around? I got a modified Baldwin amp that I believe came from an organ. It consists of the following tubes.

3 12AU7 tubes 1 labeled Baldwin 2 Electro Harmonix
2 Harmon Kardon 7408 tubes
2 Baldwin 6L6GB tubes
1 Baldwin 54UGB tube

I've been listening to it for quite a bit now and I really like the sound it makes. Lots of subtle details have come out in various music I've been listening to. It isn't as forward as some SS amps I have. The previous owner said the tubes that it came with tested well but he didn't know the amount of hours total were on the tubes. I'd like to get the exact same versions of the tubes I currently have on the amp because I'm not exactly sure how to rebias the thing. I've priced out these tubes from Brent Jessee and it is getting bit pricey for me which is why I'm thinking about getting a couple of the necessary spares first.

Thanks in advance for the advise.

Hi everyone. Thanks for the sage advice. I've been listening to this Baldwin 6L6 tube amp for some time now with the current tube complement and I really do enjoy it. It is matching well with a pair of KEF 103/3 speakers I have. Big improvement in imaging and overall soundstage. I'm getting lots of nice details in various music I've hit the amp with. I think it only ran out of oomph when I was blasting some Blind Guardian.

I'm trying to find out information about the amp now.

Here is the current tubes in the amp.

3 12AU7 tubes 1 labeled Baldwin 2 Electro Harmonix
2 Harmon Kardon 7408 tubes
2 Baldwin 6L6GB tubes
1 Baldwin 54UGB tube

Replacements/spares I'm looking into based on my current budget.

3 Matched HP 5963 Large Plate NOS 12AU7 tubes to replace the current trio. That or just 3 new Electro Harmonix 12AU7 tubes.

2 TUNG-SOL 6V6GT Re-Issue tubes to replace the pair of HK 7408s.

2 Russian Military surplus 6Pi3C tubes to replace the 6L6GB amp tubes.

1 ELECTRO HARMONIX 5U4GB to replace the Baldwin 5U4GB tube.

Please let me know what you think. I'm hoping these tubes won't be a step down from what I currently have in the Baldwin amp. Is there a formula to follow with cost of tubes vs. cost of amp?

I will definitely not change the power tubes unless someone can re-bias the amp for me or I can find info onto how to re-bias this amp myself. I was told I could change the 12AU7 tubes without re-biasing the amp. Since I'm happy with the current sound of the tubes in the amp I won't change them out till a tube dies. (I know famous last words.)

Thanks again everyone for the help.