I have seen people use 12AU7, 12AT7, the circuitry might be tweaked and it will accept anything. I use a 7025 in second stage (look into the linked thread “Once more unto the phonostage dear friend!”). Hey, I even used a transistor as follower! I know Dima has instead in input stage a nuvistor… We also run 2-stage vision of 834P with pentode 7788 in fist stage. So, anything will work in there if the circuit is properly adjusted for the given tube/s. How will it all to Sound is another story…
The 12AX7 is very common tube. If you do not have a stash of your favorite 12AX7 tubes (like none of your electronics use it for instance) then I would not urge to run and to pay $200 for a 3 good 12AX7 but use whatever 834P comes with. Since you are just trying it then it is good idea to borrow the good tubes from some of your local guy.
BTW, the 834P is quite sensitive to electricity that is able to destroy it’s sound much more then bad tubes…
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche