Hi Stuart,
I will also recommend you to stay with stock or MUTINE provided 12ax7 replacement tubes, and only when needed.
If my memory is correct, all my Audiomat amps ship with NOS
Sylvania 12ax7 and a Radio Tech 12au7's in the Solfege Unit.
I was able to tube roll with 1950's Mullards, 1960 Tele's, and even tried NOS Mullard Miltary Issue tubes, they will change the tone of the sound but I always went back with the tubes that shipped with the unit, it just always had the best balance sound with the stock tubes.
If your unit was purchased second hand, you may have to verify if your 12ax7's are stock or replacements.
Many folks describe these small tubes as "Preamp Tubes", my dealer explained to me these small tubes are for the amplifier section, one (1) tube is a phase inverter and the other two (2) are drivers. The Volume control is a primary passive design.
It is also my understanding these drivers/phase inverter tubes should lasts many+many years with trouble free use.
If all your tubes are stock and in great shape, I would spend any extra dollars at other parts of your system they may need upgrades and get back to just listening to music, you can spend big dollars on tubes and also many hours of your time with tube rolling in these Audiomat Amps, then find yourself right back where you started.
Again, Nick Gowen at True Sound would be the person to ask about how these amps operate, if you have the correct tubes, and any possible suggestions how to improve the performance of your system with the Arpege inserted. Nick will provide facts about these units.
Best of luck and enjoy your unit,
Steve