I actually answered this question in a different thread a few months back.

Here's what you need to do, referring to the schematic on
www.trinityamps.com/phpbb/viewtopic.php?t=859 :
C1 = 0.01uF
C2 = 1uF
C3 (180pF) removed
R7 = 820 ohms
C4 (25uF) remove, or replace with 1uF or 2uF if you want more gain
R15 = 820 ohms
R18, R19 = 330k (increase to 470k for more Marshally power tube distortion)
R20, R22 = 8.2k
R25 = 2.2k (don't use the choke option)
Set the OT for 8k operation. With the West Labs' OT this means running an 8 ohm speaker on the 4 ohm output or a 16 ohm speaker on the 8 ohms output. This is very important.
At this point it still has the Vox style tone stack, and you could optionally put these more Marshall-like component values into the tone stack if you wanted:
VR2 (treble) = 220k
C5 (treble) = 470pf
R34 (slope) = 33k
C6 (mid) = 0.01uF
R10 (mid) =15k
VR3 (bass) = 500k
Connect pins 2 & 3 of VR3 together.
Disconnect junction of C6 & R10 from Pin 2 (wiper) of VR3
Remove connection between VR3 Pin 1 and Ground
Connect junction of C6 and R10 to VR3 Pin 1 (so pin 1 path to ground is via R10)
And that's it!
BTW this will give you an amp with a Vox meets Marshall vibe, that's actually very close to a 65 Amps' Soho.
