All that being said, here's one rather versatile way to
get close to the earlier EVH tone. What you basically
need to do is to turn your Marshall into a High Octane
tweed Fender!
 
First, replace the EL34' with 6L6's. That's going to
make a big difference right there all by itself. 
 
Basically you want your rail voltages in the preamp
similar to that found in a 5F6A bassman. Leave the
power amp running at around 470v. That'll keep the
amp tight and give it a more aggressive quality. It'll give
you a similar effect as using a variac without the
associated risks. 
 
The next thing to do is to change the caps in the tone
stack. Swap out the 0.022 mid cap and give a 0.01uF a
try and change the slope resistor to a 47K or a 56K.
This will goose up the low end response without giving
you a fuzzy sounding distortion because it's being
implemented at the end of the preamp. 
 
Also I wouldn't really recomend bypassing the second
cathode resistor. If you do that the amp will have a
much looser feel which is NOT what you want. You
could consider lowering the cathode resistor to around
2.7k, but you really don't need to go much smaller than
that.
 
Depending on what type of guitar, pick-ups, speakers
and such, you'll probably want to noodle with the cap
values somewhat to stroke it into where you want it.
There's a LOT of variables but the stuff above should
get you pretty close as a starting point.
 
Regards;
Carl Z
Summit Amps
http://www.geocities.com/summitamps


Personally speaking I like the ceramics in Marshalls.
Silver Mica is a bit to "crass" sounding to my ears
when they are in a gain-amp. I like them in a clean
amp, like Fender Twin for example.