As a preamble, I am charged with maintaining my bands amps and tube preamps (currently six amps and three preamps) doing everything from tube replacements, re-biasing and tweaking circuits.
In the case of your amp, you have two tubes driving the tone circuit and two are preamp tube that drive the MOSFET transistors. The 12AX7 and 12AU7 differ only in their output gain with the 12AX7 being defined as 100%. The 12AU7 has a lower output of a relative 20%. In this scenario I'll hazard a guess the AX7 is the preamp tube and the AU7 is in the tone circuit.
There will be no need to have matching pairs in the tone circuit but matching preamp tubes will ensure balanced driving of your MOSFET's so I would pay that added expense.
As far as what to buy, well everyone is going to have opinions and right-wrong really comes down to what you want and what sonic differences do you hear. I personally like TungSol, Mullards, TAD valves. I have used JJ, Electroharmonix and even pilfered our campus legacy stock of original RCA and GE tubes. I would recommend going to the previously mentioned
www.thetubestore.com - Your online source for audio vacuum tubes. or
www.tubedepot.com, reading their evaluations and comparisons and match that with what you think is important sonically and what fits in your budget.