12au7 or 12ax7

I recently purchased my first tube amp a Primaluna Prolouge 5, the previous owner said the tubes were at the end of their life. I replaced the factory KT-88's with Tung-Sol KT-120' with which I am very pleased. My questions is which would give me the biggest sonic benefit replacing the 12au7 or the 12ax7? Both will be replaced in time which do you think I should do first?
Give Kevin a call, at Upscale.  No one will be able to offer better advice, regarding which will yield the most, and what tubes will best provide your favorite aural flavor.  (https://www.upscaleaudio.com/pages/contact-us)
12AU7(17db) has lowest gain and 12AX7(100db) has highest -- so I would assume that 12AX7 is more volatile for upgrades.

i think the AU's are driver tubes and the AX for the preamp
Both will make a difference but I think the most noticeable will be the pre amp tubes. But before you change anything get used to the KT120 sound first and from there decide what if anything you want to change in the sound. For me all different manufacturers tubes have a sonic signature meaning all AU/AX tubes sound different. Brent Jessee lists some of the differences but it in the end will vary a bit depending with the topology used for the different amps.
Good Luck
What, "preamp"?  The Prologue 5 is a  push-pull power amp.  It's nine pin dual triodes are phase-splitter and gain(driver) stage valves.
The 12AX7 is the voltage amplifier in the amp

The 12AU7 is the driver tube

The 12AX7, being upstream, will have the most sonic merit. The best made were Telefunken but are nearly impossible to find NOS at this point. So if you're rolling tubes, this is the place to start. Keep in mind though that the 12AU7 is going to effect things too- whatever you hear from the 12AX7 will be passing through the 12AU7.

What I always tell my customers- have at it, have fun, and don't drive yourself off the deep end.
Current version Gold Lion 12AX7s are great tubes, and after trying various new and NOS options in my amp I'm now sticking with 'em. Note that the driver tubes in my amp are 12AT7s and I found that NOS Mullards not only sound great but "flash" when they start up, which illustrates my low threshold of entertainment. I used 4024s but switched to 6201s since they have gold pins…I like the gold pin thing in my gold plated (I think) tube sockets as that's one place I prefer to have proper metal contact…note to self: Get the tiny brush tube socket cleaner thing. Does anybody make gold pin power tubes yet?
@wolf_garcia , be careful with those gold-plated tube sockets! Gold is soft and so the socket won't survive many tube changes before the plating fails. When that happens the raw copper beneath is exposed and the socket contacts will turn black.

IMO/IME, you are far better off with tin-plated sockets, as tin does not want to corrode (similar to gold in that regard) but its much harder, and so the socket can survive for decades of use.
The sockets came with my relatively inexpensive Jolida 502p so maybe they’re NOT gold plated…but if they are I now wonder if I should replace them…hmmm….soldering is what the young girl wants…also I have to wonder what "many tube changes" means as I don't mess with 'em THAT much...
My new favourite 12ax7 tube is the Shuguang Custom 12AX7 LS  @ $99/pair

Impressive sonics. You won't be disappointed.
would agree with atmasphere. That first tube (12ax7) will likely have the largest impact on sonics.
Another option is the 12at7 which will lower gain a bit. but to keep everything in balance, you should change out the 100k plate resistor for a 33k, and the 1k cathode resistor for about 500 ohm resistor. This will allow the plate voltage to come back up to speck for the at7.

You can try the at7 without doing anything, but the plate voltage will drop all the way down to 50-60 volts or so, without the resistor change.

On any of the PL amps, upgrading the coupling caps, helps tremendously.