No since you are using 6l6's doesn't mean you can only use 12ax7's but honestly I wouldn't use any other preamp tubes in the preamp of a high gain amp. You could use 12at7's which would lower your gain or you could use 12au7's but those are more for driver and phase inverter positions.
12ax7's are the king for almost all guitar amplifier preamps.
Tubedepot online is a great place to call or email. The tube thread is good as well but can be very very detailed. If you can get to the bottom of all that detail. :wink:
