The feedback and presence circuit are one of
those circuits that with proper design, act like
a master tone and gain control. 
There has been some real good posts here
about the voltage ratio here and if you look
back you'll see how this is effected by
swapping parts around. 
An important thing to remember is the
resistance under the PI to ground when
compared to the feedback resistor. 
In the black face Fenders, I've been
pondering an experiment using a 25K L pot
but connecting the wiper to the junction of the
bottom grounded grid cap and the 6K8 to 22K
resistor at the middle of the PI. The top of the
pot would back to the OT and the bottom of
the pot would go ground through a fixed
resistor cap combo. 

That very high voltage in the PI of the black
face Fenders in conjunction with those
feedback resistor values, probably has a lot
to due with the sound of those amps. 
If you like the real clean with lots of head
room sound then keep the B+ way up there. 
But if you like it a bit darker, less headroom
and loose around the edges then drop the
supply voltage down to under 350V and drop
the feedback voltages to boot. 
I think that section has enough gain to drive
the output section into distortion at 300V with
a 12AT7 and more with a 12AX7. But the
sound you'll like is all the preamp sections
fuzzing out a little more when turned up to
compensate for the lower drive. 
After all this, it won't be a typical Fender
sound though.