What I have found with my Friedman goes true for most Marshalls as far as the preamp tubes go. My favorite in just about any amp I have had are the old Tesla E83CC silver pins, and the Siemens E83CC triple mica. The damn things last for years and sound incredible in just about any amp I have used them in. That's what is in my JMP. I have tried just about all of the old tubes. I used to collect them. Below was the rarest I ever had. I sold that quad for 800 bucks. They were unused and even had the same flashing patters. All same XF2 Amperex DD getters. These were my favorite EL34. The old Tesla Brown bases are great for guitar also. The only Mullard I liked for guitar were the chocalate brown base XF2 single getters. The double getters were too high Fi. I have a quad of unused original Tung Sol 6550 and GE 6550 I am going to be trying out in my Marshall.