12au7 or 12ax7

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.
http://www.audiotubes.com/12au7.htm
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.