Marshall Super PA - mod suggestions?

[shakti]

I have an itch for a vintage Marshall again after building tons of clones over the last 10 years, and after owning a lot of amps it has boilt down to the JTM45/100 and a '69 style Super Lead for me as the two essential Marshall 100w types. Finding an original JTM45/100 is out of the question for price and availability reasons...and even an original 69 would be too expensive these days. Therefore I am looking at picking up a Super PA as a base for a "mod machine" type amp.

Bearing this in mind, what I am looking to achieve is to have an amp that is 100% a '69 Super Lead in circuit, including the normal channel. This is very easy to achieve with a Super PA, just a few changes and presto - SL specs. But there are a few valid circuit variations that I want to have the ability to switch between. The amp I am looking at is not a virgin amp at all, it has apparently been modded before and there is an extra hole both in the rear and the front, so I have no second thoughts with using it as a base for more extreme modding. I am looking for suggestions on what else I can put into a "switch machine" type amp, without messing with the core of the amp too much, i.e. be able to have 100% 69 SL specs as well as other options.

I am of the opinion that the pots are crucial to getting the vintage tone and EQ, so I do not want to change any of the EQ pots, and as few of the volume pots as well. That limits the options of switchable pots somewhat, but there are still four volume pots on a Super PA which does give some possibilities.

What I am considering at this point:

- Lar-Mar PPIMV
- Variable negative feedback, possibly all the way from 27k/16ohm, with a 250k pot in series so it will cover all the ground up to 100k/4ohm levels as used on metal panel amps and JCM800/2203s.
- Tone stack switch; keep the stock 250pF/56k on the board, and be able to switch bass/lead tone stack by parallelling in another 250pF and 82k
- shared/split cathode switch, so I can get as close as possible to a Super Bass type tone as well (will have to compromise on output couplers which will be 0.022, and will have to figure out how to get 0.022 first stage coupler as well.)
- V2A bypass cap switch - to add a .68uF. These were not present in JCM800s, so I would possibly like to have it switchable
- Vol 1 Bright cap switch - possibly

Since the amp has an extra preamp tube, I want to add more of a modded SL type circuit as well. But since I want to have 100% Stock SL specs incl the normal channel, I thought I'd use the extra preamp tube to have 100% JCM800 V1 specs on one tube, and 100% 69SL on the other. So a switch to go between 69SL (on, say, channels 3/4) or JCM800 (on channels 1/2). This means I would have to plug the guitar into the corresponding channel to go to JCM800 mode, but as far as I can understand, this is the only way to do it if I want to keep 69SL specs 100%. The typical 2-in-1 mod loses the normal channel in order to switch to JCM800 mode without plugging into another inpuit, but that's not really a loss for me. I'd rather have 100% SL specs.
To switch between the two modes, I thought about using a 3PDT switch so I can switch the three points in the circuit where the amps diverge;
1) cutting off the B+ to the tube not in use, to keep it 100% out of the circuit
2) switching the input grid of V2A
3) switching the output from the EQ section into the 0.022 PI input - either directly from the treble pot wiper or via master volume in JCM800 mode

If I do the switching as I outlined above I can keep the amp 100% SL specs and keep everything JCM800 about it entirely out of the circuit, but it may be overkill to do the switching this way? What do you think?

Any other suggestions for valid/useful mods that I can implement without messing with the 69SL specs?