The difference the Mark Moyer mod makes on a Fender Custom Vibrolux Reverb is astonishing. The mod involves removing the reverb from the Normal channel, lowering the gain of the pre-amp, increasing the caps on the reverb to make it fuller, replacing the tone stack capacitors with higher quality caps, removing the protection diodes on the 6L6s, adding negative feedback, and changing the ground reference resistors on the heater lines.
The mods really open up the sound of the amp and give it much more clean headroom. The distortion changes from a pre-amp based fuzz to a power amp crunch. The reverb has a lot more depth and realism. The mod also takes care of the constant hiss in the amp.
Some versions of the mod suggest swapping V3 (reverb driver) 12AT7 with V6 (Phase inverter) 12AX7 to improve the dwell of the reverb and decrease the gain of the phase inverter. While some classic Fender designs did use a 12AT7 for the phase inverter, none used a 12AX7 for the reverb tube. The 12AX7 is not suited to drive reverb. It will not damage the amp or tube in any way, but the 12AX7 does not have the needed power to properly drive a reverb tank. I found the tube swap made the reverb sound lifeless and it took away the sparkle of the overall tone. Feel free to experiment.
JDP Amplifiers 
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