The Valve Reactor used a transformer with the tube as a mini-power tube and then amplified that up for the output. The Valvetronix did away with that and just use the tube as a starved-plated preamp to help filter the sound to be more tube-like (check the Valvecaster DIY pedal for inspiration...it's not a bad thing). It is not like the the Reactor and swapping will not have as much effect. Not sure which system your amp used. It's only a 12ax7....swap it and see. No need to go Telefunken or Mullard...find a cheap, common one like RCA. If it sounds better to YOU, then it's worth doing.