my 2 cents
i don’t have this particular luxman pre... so this is from general knowledge and a lifetime of tube rolling
12au7 and 12ax7 are usually NOT interchangeable... vastly different gain characteristics (that is why those three tube types were designed and made in the past, despite the common 12v heater dual triode small envelope designs -- 12au7 vs 12at7 vs 12ax7 - very different gain profiles from one to the other -- 1x 3x 5x gain, thus applicable for different circuits)...you can bet that putting in a 12ax7 with 5x the gain of a 12au7 in your nice luxman will create problems
you would need to contact luxman, or a knowledgeable dealer or tech to confirm specifically which three 12au7 slots are for main amplification, not tone control buffering circuitry (what is drawn on the circuit diagram is idealized, the actual circuit board may not be laid out in exactly the same fashion)... you CAN safely swap in different 12au7 tubes in those 5 positions, but without knowing for sure, you won’t be able to assess what you are hearing in the attempt to tube roll
here is a fairly complete discussion and listing of 12au7 equivalents (these are different names for the same tube, from different parts of the world, or very very similar variants made over time i.e. improved ruggedness etc etc - with identical or very similar gain profiles) - you would be well served to read it carefully and completely
http://www.audiotubes.com/12au7.htm
