str420's are a poweramp 6l6 tube and yes you can swap any 12ax7 over i personally think the mesas pre's are crap and any other pre's ive put in souned better and had a lot less noise and feedback
ive got a dead mint condition mesa mark4 combo looking to trade for a soldano avenger or a really old dual recto.im in perth western australia though pm for pics
i had a series 2 single ages ago and i had a 2 channel dual which was way better than it a lot of people say that the modern channel is very similar to the 2 channel dual