If its an older amp updated to Mk3, the tube layout might be different.
So, in the latest iteration, the tubes in the rear beneath the meter on either side are the most important to the sound. The one in the middle is next most important.
In the very early Mk3.0 (long chassis) the most important tube is the one in the front, followed by the one right behind it.
In older amps updated to Mk3.0 (and up to Mk3.2) that have the more rectangular power supply cover, the front left tube is most important, followed by the front right. If updated to Mk3.3, both front tubes have equal importance.
The driver tube sits by itself on the later chassis (with the more curvaceous power supply cover). It should have either a -GTA or -GTB suffix. If the long chassis version, this is the rearmost 6SN7. In the older rectangular style, this is the rear right hand 6SN7. While this tube has less effect than the others, it is still audible.
One thing to keep in mind- you may run into intermittent operation when trying different tubes. We use the Chinese 6SN7s, which have a longer pin on the base of the tube that is also slightly larger diameter. So the socket pins might be spread a bit and have a looser contact with NOS tubes which often have shorter pins. The sockets are easily tightened without taking the amp apart. Contact me if you run into something like this and I'll walk you through the process- it only takes a few minutes.
