ALL my Fender reverb combos have either the vibrato
circuit disconnected or switchable via that Mouser 50K
switch pot. I think it makes a very noticeable
improvement in the depth and clarity of the tone.
 
I'm also a big fan of replacing the ceramic caps with
silver mica. It definitely takes some of that harsh
graininess off of the top end.
 
Here's something else you might try:
 
1) Replace the .1uF and .047uF caps in the tone stack
with .047uF and .022uF, respectively. (In a SR, unlike
most other models, the middle tone cap is already .022,
so you can leave that one alone.) Also, make sure the
slope resistor that feeds these caps is exactly 100K; if
it's an old carbon comp that's drifted up to 110K or
more, replace it!
 
2) If your amp doesn't have a middle pot, replace the
fixed 6K8 resistor soldered to the back of the bass pot
with an 8K2 or 9K1 resistor. If your amp does have the
middle pot, you might consider replacing it with a 25K
pot for more midrange capability.

Look, I can see using 1/2-watt carbon comp in the
signal path. Lotsa folk like the little bit of grit that they
add, and they'll hold up all right for a good while. But,
it doesn't make any sense at all to use carbon comps of
any rating for plate load resistors, as long as you have a
choice. They may work OK now, but they'll be
cracklin' up a storm before you know it. 
 
Shea
 
3) Replace the ceramic .001uF cap (blackface value)
that's used as input to the phase inverter with a .0039uF
or .004uF film cap. Make sure it's a good quality one,
such as an orange drop.
 
These component changes give a smoother and fuller
midrange, and goes a long way in taming that typical
Fender "tubby" bass, scooped nasally midrange and
tinny treble. Combined with the above vibrato and
ceramic-to-mica cap upgrades and applied to the
blackface AB763 circuit, these mods can really make a
Fender reverb amp come alive. After your ears get used
to this sound, I predict it'll be hard to go back to the
stock blackface tone without feeling like something is
missing.


I came up with the exact tone stack mod on my own
that you're talking about.
 
As far as the coupling, my Twin is a SF, so I went from
.01 to .001 - one extreme to the other. I think I need a
little more bass now, so the .003 - .0047 sounds like a
good idea.
 
Another thing I did - I put a total Marshall tone stack in
the normal channel, and connected it to the 2nd gain
stage instead of the 1st. I like it.
 
Shea