Strange...switch cleaner shouldn't nuke the volume-pot (used some recently on my II as well) and I find it unikely that both op-amps have gone at the same time :?
Did you try switch-cleaner on the source-selector mechanism? mind you, again it would be unlikely that all inputs don't work even if one or two needed it. I assume you had the amp disconnected from both the mains and the PSX when you cleaned it?
It's a pitty you can't get hold of the fuses to take the PSX out of the equation (even though there's no reason why It shouldn't still be working). The fuses are 2 x 20mm 3.15A quick-blow fuses, which you can buy in packs of ten from Maplins for a couple of quid.
When you say it crackles when you attached the interconnects, I take it you mean only wth the volume up a little, not with it down?