Board A and Board B alone are dead silent when powered solo
cause I can hear the noise set in with no input connected the moment both boards are powered.
You mean if you apply power to one channel it is quiet, apply power to only the second channel it is quiet, but when both channels are powered they are noisy?
You said you do not have access to an o'scope to check the power stability, correct?
I don’t know where a ground loop can still possibly exist
The term "ground loop" typically refers to the case when different pieces of equipment have current flowing through the cables, and because of incorrect connection of shields and power earth connection some of that current flows through the audio circuitry. If you do not have other pieces of equipment connected when testing it is not likely that the source of the noise is a traditional ground loop. You seem to be inviting problems by still insisting to connect all the different boards to the chassis, but without some voltage difference to drive current from the chassis connection and out a different location it should not cause problems.
I should note that it is not entirely clear what exactly you are referring to when you state that "All the boards are grounded to the star point independently."
Do you have schematics for these boards, so you can point out exactly what node you are connecting to the chassis? I suppose it is still possible that there is something that could drive circulating currents between the two buffer boards, but
Given your latest information that (if I understand correctly) with no other equipment connected, powering one channel is quiet, powering two channels is noisy, it seems power supply problems are the only likely explanation.
Since each channel has rectifiers and filter capacitors on the same board as the buffer amp it is not a case of the power supply capacitors being undersized for the current and causing a lot of ripple when two channels are connected, so the next likely cause would be exceeding the current rating of the transformer. That would usually just cause a lower voltage and increased transformer temperature, but perhaps the extra magnetic field strength inducing a voltage into some of the wiring.
I looked back through the earlier posts, but I did not see anywhere a specific model number of transformer used. How confident are you that the transformer is actually appropriate for the current load of both channels?
I’m putting all my eggs now in the external rectification basket.
You don't have to necessarily go to external rectification, leave the diodes on the power supply boards, but move the transformer out of the chassis as a test. Put on some temporary extension wires from the transformer secondary windings so you can move the transformer away from the chassis.
today I got the bridge rectifier so that seems to be my only hope now.
What are you going to do with an additional rectifier? What specific problem are you attempting to solve with that, and why do you think it will help?
There are some cases where an external power supply can help by getting the magnetic fields from the transformer farther away from the circuitry and wiring, but in that case you would need rectifiers and capacitors (and possibly chokes and resistors, depending on how you were splitting up the filtering).
If the capacitors are still going to be in the chassis you might as well just run AC to the chassis (from the secondary windings) and leave the rectifiers on the power supply board.
One big thing which just jumped out at me is that I cannot see where the input transformers are in your original pictures. I see two transformers on each buffer board (SRPP and power supply PCB) both labeled "Output Transformer." Is one of those being used as an input transformer? I zoomed in on the pictures, and the wiring is confusing to me, I see four terminals near the transformers, but one of them has two twisted pairs connected. I would expect one twisted pair to each terminal, a pair on the transformer primary, and a twisted pair on the transformer secondary. Really need a schematic to see what is going on.
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