Time to tinker?

Or should I just go with "if it ain't broke...?" I do like the sound of the amp at present.

I've got a drawer full of preamp tubes, and I'm wondering if any of these would help in the hitting of GL's sound a little better that what is stock in the amp.

1. now ANOS Telefunken 12AX7 flatplate grey -- this is balanced
2. NOS JAN GE 5751 -- this is balanced.
3. NOS prewar Ei 12AX7 flatplate -- untested
4. ANOS Mullard 12AX7 -- pulled from barely used Fender amp before sale
5. Tung Sol 12AX7 (new)
Assorted JJs, Marshall Shuguangs

I'm thinking V1 and possibly the PI. Choices? Ideas?

As I have mentioned in other threads, I have tried tons of preamp tubes in the modules and I can not hear any discernable difference if working within the same family. That being said, there are two other places you may want to try a preamp tube swap.

First, the 12aX7 in the amp chassis that is directly above the input jack is the first preamp tube that your guitar signal sees, the first gain stage before the signal makes it into the modules. If you are shooting to hear a tonal difference, I would guess that this tube has the most control over tone.

Second, the 12Ax7 closest to the power tubes is the phase inverter/driver tube. It is a driver tube for the power tubes. This tube can have a pretty dramatic impact on the sound. I installed an Electro Harmonix 12AT7 in this position to lower the gain a bit here and keep the signal clean as it entered the power section and I DID hear a discernable difference. To me, running E34Ls in my RM100, this change made things smoother, a little less harsh and brittle.

So again, these two tubes are probably where you could find pay dirt. Just my 2 cents. Rock on.