Here's a quick one that'll make a DRRI distort more easily, and is reversible if you don't like it.
1) Looking at the amp from the back, take out the rightmost preamp tube (12ax7a), but don't throw it away, you'll need it for step 2.
This will render channel 1 inoperable, but will send more input signal to the vibrato channel.
2) Take out the Phase Inverter tube (12at7). This is the last small tube before the power tubes, coming from the right. Replace it with the 12ax7 removed in step 1.
This should further reduce the channel's clean headroom.
If you prefer channel 1's sound and don't need reverb and vibrato, you can apply step 1) to the 2nd 12ax7 tube from the right instead.
With PAF's, my DRRI starts getting a nice crunchy OD around 5 on the volume, and from 7-10 I begin to feel like Neil Young. A nice clean boost in front (Xotic EP boost in my case) is my "channel switch".
There's something about Fender distortion that is so distinctly seventies' rock 'n roll to me...
"Recording exactly what a musician hears turns out to be a really big deal." Bob Olhsson
"Who cares about efficiency, when we're talking about music?" Rupert Neve
"it'll sound different through a microphone, anyway" Keith Carlock
"no room, no boom!" Michael Wagener