To avoid confusion, we call em driver tubes ...
You can use any 12AU7, 12BH7, 5814, 5963 or 6189 as a driver in any of the positions. Also any of the "A" variants. The center tube is the first amplifier stage and it's there you'll get the most bang for the buck. Good thing to have a triode balanced tube in that position, and a decently matched set of tubes in the side sockets. Not as important if you have balance controls to even out any inconsistency. Those two should be the same brand and year, better yet, same production batch to keep the "flavor" consistent in both channels, but they don't have to match the center tube.
Have fun with substitutions. I'm currently running a circa 1940's RCA foil getter 12AU7 in the middle and a matched pair of Sylvania 12BH7a's in the sides. The Sylvania's tend to be a bit dark, and the RCA center overly bright, but together, they're just right.
PS ... the older 12AU7 tubes without the "A" designation on the end are argued to be more microphonic than the newer varieties, but that hasn't been my experience. Then again, all my stuff is "military grade" which are built stronger and more resistant to vibration. Those will be identified with some sort of suffix printed on the tube ... 5 Star, JAN, JW, or some such, depending on the brand.
Just be careful though ... tube rolling can get addictive ... Now excuse me, I'm late for my next fix
Addendum ... you also asked about internal mods ... maybe not a good idea as it will limit your options. I hear you can also use CV181 using a 6SN7 socket adapter, but the bottles are too big to run three of those, and they're quite expensive. Not sure about using the actual 6SN7 tube in our amps - no real world experience I've heard of using those.