Question on tube placement...

I currently use all tube gear for my home audio set up (SupraTek Chardonnay Line Preamp, Decware SuperZen Monoblock power amps, Cary Audio PH-302 MkII Phono Pre and these run through a 1968 pair of Altec Lansing Flamenco Speakers).
My Line Pre and Mono power amps have (through much trial and error) a pretty diverse range of tubes in them. Mostly NOS, but a couple newly manufactured gems.
The Cary Phono Pre is my latest purchase. It’s only about 5 months old at this point. It’s tube complements are as follows: 2 ea - 6SL7 Head Amp, 2 ea - 6SL7 RIAA EQ & Amp Gain Stage & 1 ea - 5AR4 Vacuum Tube Rectifier.
I have already replaced the rectifier tube that came with it with a late 50’s Mullard Blackburn 5AR4/GZ34.
I want to replace the 6SL7’s, however, I will only be buying two at a time (due to cost). I plan on buying two Shuguang WE6SL7’s.
My question is which part of the chain do I place these two in for the most impact while I save up for the other two? In the “head amp” part, or the “Amp gain stage”?
Thanks in advance for any advice...
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@jvankamp78- Audition a pair in each position, and see which you prefer. Don’t miss the(NOS) Tung-Sol(VT229), Round Plate(probably the premier 6SL7): They’re not nearly as pricey as the 6SN7 variant. ie: Should you want to warm things up a bit, add a couple Brimars, somewhere in the chain, Here’s a link to some opinions(keep in mind- system dependence/grain of salt): Great pick(the Mullard/Blackburn) for your rectifier, btw!
My first sentence should have said, "Audition a pair(of whatever you select) in all positions and see where you prefer them."    Take your time and really(critically) listen, before moving on.
@rodman99999 Thanks so much for your reply. Lots of great info and great links!
Have fun and happy listening!
IME its best to put the best tubes as early in the signal chain as possible. Of course this asks the question 'which tube is best?' and if you are messing with NOS tubes that can be tricky to answer- some tubes work better in certain situations than others. Of course, the tube should be low noise and low microphonics in early gain stages. It doesn't help how good the reputation of the tube is if it won't pass tests...
@jvancamp78- I don’t believe anyone is more conscientious than Brent Jesse(the site to which I referred you), regarding the testing and performance of the tubes they sell. As comprehensive as you were in your initial post; I assumed(UGH) you’d mention those potential(high-gain) positions, whenever contacting a tube monger(ABSOLUTELY: a critical conversation).