I totally agree that the problem with this amplifier is being caused by 'modification' of the unit from the original well proven design in that the original smoothing cans have been removed and the replacements that are mounted below deck don't have their negative/ground connections returned to chassis correctly and as of the original design.
There is absolutely no hum problem with this amplifier as built in its original form - I should know, I've had one for decades.
The first question that I would ask is how did it perform originally before all the re-capping? I'm going to guess that you don't actually know because you didn't try it before doing a blanket replacement of capacitors, but I really hope you're going to tell me I'm wrong on that guess.
The paper capacitors that you've replaced so far certainly do need replacing in this amplifier if they're the original type. The electrolytic capacitors in the cans may well have been ok to leave, although in my particular amplifier I did have the smoothing section of the large can develop internal leakage causing burning of the associated resistor and warming of the can, so that had to be replaced.
To keep this valuable and desirable amplifier authentic, you really need to replace the smoothing cans with proper replacements, mounted as original on top of the chassis. Leaving holes in the top where they've been and fitting single replacements below deck will ruin this amplifier and destroy its value. Having said all that, I will admit that on my particular amplifier, when the smoothing section of the can failed, I did a temporary repair by isolating the bad capacitor section and mounting an axial type beneath the chassis - and that was perhaps nearly a decade ago, but it's my amplifier and I've no intention of selling it or passing it on to anyone else in my lifetime. If you're repairing your amplifier to sell on or even repairing it for someone else, then you need to do the job properly, unless of course, the person wants a cheap job done and is happy with having holes left in the top of the chassis.
To stop the hum problem I think you either need to return the negatives of the replacement electrolytics to as near as possible to the original design and reinstate that missing bus bar connection, or fit proper direct replacement capacitors, also reinstating that bus bar line.
Below are the original drawings of the amplifier layout, and the first one says it all as regards the connections - no extra chokes needed. This was a proven design, ok, designers do get it wrong sometimes, but my advice would be don't try to modify their design unless you have the technical knowledge to do so
