From Liam on Marshall Forum:
http://www.blamepro.com/CGI-BIN/cutecast.pl?forum=31&thread=76

Grev put up some stuff to help people with biasing, so I thought I'd
whack my mods up here to, for future reference.
Firstly, the standard warning:

ALWAYS discharge the caps in your amp BEFORE doing ANY work inside it.
If you don't, you might not live to hear your modded amp!

If you don't know what you're doing, DON'T DO IT! Get someone who does
to.

Ok, this mod is very simple, cheap, and best of all, it's easy to undo
if you don't like it.

It gives the amp more gain, bass and mids, and gets rid of a lot of
that ear piercing treble and brightness the dual reverbs are notorious
for. It also seems to smooth the grittiness of the distortion, and
make the amp sound more full and punchy.

The following is a list of parts to change, and what they do.

Just a note: The PCB's in these amps don't take to kindly to rough
handling. The copper etching comes off very easily. So take care when
desoldering components.

R10 (100K) - change to 220K or 470K
gain boost (higher resistor value = more gain) I ended up using a
220K.

C8 (470 pF) - change to 100 pF
Stability

R37 (22K) - change to 10K
Gain boost

R7 (4.7K) - change to 10K 
Pre-distortion mid boost.

R55 (100K) - change to 220K
Reduced loading, more effective gain

R15 (100K) - change to 220K, in parallel with that, run a 100K
resistor in series with a .005 uF (.0047) cap.

This gives a post-distortion bass boost. You can change it to suit
your liking. The 220k resistor sets the amount of boost (higher
value=more boost) and the cap sets the frequency that is boosted. I
originally used a 470k resistor, and the bass was really flabby, and
boomy. So to cut the boost back, I used a 220k.

You'll have to add the series resistor/cap combination external to the
PCB.

Something like this

|----|100k|--|.005uF|---|
|---------|R15|---------|

Extras:

You can also change your tone slope resistor (R61) from 33k to 56k,
and the treble cap (C37) from 470pF to 250pF. I had already done this
to my amp before I did the rest of the mods. It also helped to cut
back highs, and add mids.

You can also change around the values of R9 and C7 on the clean
channel. I haven't done this, so I don't know what it sounds like. If
anyone has, how did it go?

That's pretty much it.

I hope I haven't left anything out.

I've done this mod to quite a few DR's now, and everyone has been
happy. As with all valve amps, the DR's need to be cranked to get
their best tone. Otherwise, they will sound thin and yuck. Just as a
Plexi does!