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One sold through here in 2015 for around £1150 (which is a very good price in my eyes!) -
http://www.thefretboard.co.uk/discussion/46142/fs-marshall-super-pa-1969-vintage-amplifier-100w-1150ono
Here's a few older ones, hence more expensive than yours:
https://reverb.com/uk/item/6241-marshall-super-p-a-1967-clean
https://reverb.com/uk/item/1354000-marshall-jtm-100-super-pa-amplifier-1967-black
https://reverb.com/item/1405032-marshall-super-pa-1966-black
Finally, I've got an early one up for sale that has the white back panel which I've stuck a high price on as I can't bear to part with it (sounds too good and looks too good in the showroom!):
https://reverb.com/item/4577105-vintage-1966-marshall-jtm-50-super-pa-mk3-iii-valve-amplifier-head-serviced
Hope this helps?
The IEC socket conversion is just what it is - when it was done, the TAD replacement wasn't available, and the new socket is still safer than the original. The only thing I don't really like is that the mains voltage selector appears to have been left operational, this should have been hardwired at the same time.
The replaced power valves are Zaerex labelled RFT, AKA Siemens, which are *very* good valves - they would have been NOS even when the work was done, they're from the early-mid 80s at the latest. The Brimar preamp valves may be original, Marshall used these as well as Mullards, and later RFT preamps too. If not then they were also old when fitted.
Be sure to check that the bias is OK, since failing bias filter caps are a common problem - actually more than the main filters - and if they die it will cook those nice EL34s. The usual symptom is that the bias trimmer has to be turned to the max to get just about enough bias voltage, which that does look like it is.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
What's involved in that mod to have 2 channels as super lead and 2 as super bass?
They are not modern Master Volume amps. To make then distort, you turn them up loud. But don't, unless you've connected both speaker cabs - and probably still don't if you value your hearing…
The reason they are so valued is because you can turn them up like that in a studio, or if you use an attenuator to control the volume - and for that wonderful clean sound, which takes pedals so well.
Adding an extra cathode cap and resistor, changing the coupling cap and adding two bright caps (on channel 1), and adding one bright cap (on channel 3).
None of this will make it distort at low volume, it just changes the voicing.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Jacking the two inputs together will provide the best of both worlds with the aid of a simple pedal jack cable. Blend them well and punch the volume on a full stack will provide some excellent drive and saturation. But then you could just have an amp with a remarkably clean high volume.
As for checking the amp out I would trust no one unless you the forum can back them up or you have reliable feedback. I am now on my 4th amp pedal guy. The last three were all recommended and seemed reliable but all 3 let me and others down. The 4th is probably the last local one we have here in Bologna and my friends and I are slowly testing him with some small repairs. So far so good and more importantly quick too.
Ideally you're after someone who can confirm that the amp is fine and give you a diagnostic report for your cash.