With the three gainstages that are involved you have ample capacity for distortion if that´s the name of the game.
First up though if you plan to lightly modify that unit... The tonecontrol of it is what´s known as a James Network. This,when setup right,is a REAL effective way of handling things.
I therefore recomend you to download a program called Tonestack calculator from Duncan amps. This is a nifty little program that lets you iterate changes to hearts desire on the computer and thus it will give you a rather fair idea of what to expect from a change.
Then. What you can do here,if you want to keep the basic character of the amp,is to play around with mainly two things. Voltage rail voltages and plate/anode resistor size. The amp uses 100k anode resistors stock and to increase these to 220k is certainly doable and will have a rather profound impact on how the amp sounds and behaves. If you want to alter the actual character of the amp in turn,that entails replacing cathode resistor values.
Rail voltages in turn. These are the resistors located at the low right on the schematic and by increasing the values of these you will lower the running voltage for the variuous triodes which will in effect reduce headroom of the amp and make it more prone to distort. For a three stager like that i´d shoot for a rail voltage around the 280VDC mark or so ahead of the first two triodes. However the reverb is run off of the same tap why i would separate the two as well,that way getting way more control of what happens at the first two triodes.
If you in turn want more "oumph" out of the amp try installing a pair of zener diodes of 30-40v/5w in series with a 100 Ohm resistor for g2 of each powertube. Reset bias accordingly. These amps normally sport rather high B+ voltages being EL-84 amps for what it´s worth.
Already mentioned is the fact of the stock coupling capacitors,normally of Atlas brand. That you have replaced these is a GOOD thing. Being of age those that do not leak will...eventually,why one should always replace anything that says Atlas. For British made amps the same goes for anything that says Hunts and for Italian ones in turn anything that says Ducati.
If you feel the need to,as distortion increases,reduce bottom end of the amp replace the stock small e-lytes across the cathode resistors with ones of 4,7uF. In turn you can try adding one for the concertina driver stage too. If a master control turns out to be needed install one between the driver and the actual concertina. 1M Ohm log does it.
Keep the 470k-1M stopper though and have that installed on the inbound grid AT the actual socket.
Then. Just as with the Fender amps that 3,3M resistor is rarely needed. Feel free to replace that one with a 1M or similar and try the amp out. What sets the bottom line is how much reverb function you feel is needed. For real hardcore "dirty" (read-distorted) work that 3,3M resistor can very well be installed on a mini switch. That way...either resistor value of choice or completely jumpered. This will have a rather great effect on distortion capacity as well.
