IC1 was indeed the culprit.

Bought from Digikey,

PART: 296-1403-5-ND
MFG : Texas Instruments / MC1458P

Soldered in an 8-pin socket, and soldering was perfect, looked like a machine did it. 0 sound. Desoldered without thinking, shoulda used a multimeter first. Went to solder it back again and a few of the traces were gone. I will mention, this board was worked on before... looks like newer jacks, the pots all replaced with cheaper ones sadly, and IC1.

Soldered in small wires to rebuild the traces, this time, multimeter like I should have done, and works fine. Bit buzzy... grounding issue, but may be in part due to this house. 100+yr old house, still needs to be rewired, no ground.

In anycase... by the time I finally got this all done, the bass pot was starting to get stiff again, so I suspect by Spring, I'll be replacing the pots with genuine Marshall. I'd use Bourns or CTS but the middle lead is offset... though still, if leads long enough, may be as simple as a small bend and it'll reach. 

I was on the TAYDA site, pretty good site, saw LM1458N's listed... any chance those work? or would the MC1458P's be a better match?

On another site someone said to swap the 1458's with TL072's because lower noise... but honestly don't know enough about IC's to know if a good or bad idea.

Then again, if installing 8-Pin Sockets... would it hurt to try both? or best stick with 1458?

Typed in 1458, MC1458 and MC1458CP1... nothing shows. Sold out I guess.

Got pics... looks like the schematic said CA1458, but chips are Motorolla MC1458CP1. Some on eBay, cheap but China and don't have the 15-45 days it takes to ship.

Do they have to be identical? I heard they can be finicky... but, I see some on sites like digikey, mouser and tayda that are close... like LM1458, or MC1458L... those work?

https://i.imgur.com/MeDbj2y.jpg

Unrelated (kinda) but, the pots... do they say Alpha on the back?

Just curious, had to clean them and the Bass was seized pretty tight, turned but with effort... Deoxit'd them all, no longer dry, and after 6 flushes the Bass is normal again. When opened it up saw they all said Alpha, which I knew made compatible pots for Marshalls, but the originals I suspect were replaced? Didn't originally say Alpha did they?

Part of me wants to replace those while I got it apart. Shame but I don't think Bourns fit. If I recall, the leads are staggered a bit differently on Marshall pots.

Nephew replied finally, pre-existing issue... I'm outta the dog house for breaking his amp. For the price, I guess there are three CA1458's and one TL701... I'll buy them all, maybe replace them all and add the holders. For the price it saves me some time... though always the risk of damage.

Thanks for the info...

I was debating, if having to do all this work, maybe soldering in IC Holders... less chance of damaging the IC with the soldering gun, and in a case like yours, if needed done again, it's a simple pull one out, pop a new one in.

Shouldn't be any downsides to doing it that way, I assume?

Hey guys... sorry for the resurrection but, thread is 99% exactly what I have happening so made more sense to reply than start a new thread of the same thing with a link to it.

Basically, identical issue as the OP, except, it's the preamp volume causing issue not the master volume. Otherwise, same problem, and even the same LED1 and LED2 light situation.

Would it then likely be the same IC that needs replaced?