Mods On A Marshall 3005 lead 12

Start by swapping op-amps, you can use all the op-amps that are used with Tubescreamers and they can all be found on ebay,.
Change the pots to mil spec as the replacements have a life of about 5 years. I use 3 turn pots that let me dial in the tone allot more precise. Same exact amount of resistance on the eq just it now takes 3 turns from 0-10, I use my ear to more finely dial it in where I want.
I wire the line outs to an American made Iron core transformer that knocks all signals down to 150 ohm. balanced out and I add the XLR on the back. This makes a world of difference in the amount of hum I get when I use the XLR to go into a DAW. Those are the beginner mods I have been doing for 20 years on these amps, if you want to move on to pro mods you can change the EQ by varying the pot values and you can always change the filter caps in the power supply section to be on the safe side, since they may be from 1985. The jacks need to be changed on older units and again using the same jacks they used is going to have a very definite life span, I would venture into using some high quality panel mount stuff and mount the pots to the board with wire so you can float the board in the chassis and use any replacement parts you want as opposed to being stuck with specialized PC board mounted NOS parts. I have seen the same pot values used on almost all the amps made and I tend to think they had been chosen on the lines of what the factory had tons of and not what the circuit warranted. Then again I could be a little crazy from playing loud guitar all my life, I am not by any means a trained electronics engineer, I am just a tinkerer.
My short list of favorite op-amps:
TI MC1458p (The original)
LF353n (a higher quality amp that some people like)
RC4558p (typcal TS type op-amp but purists want the JRC only.
JRC4558p (the original TS-808 amp)
http://marshall-amps-guitar.com/