Just an update to clarify that the noise starts once the T6Pro turns on the audio output. Once the receiver knows there is audio to be played, there is an audible click on the pre-amp and the output starts.
I guess there is a relais-shutter inside that turns output on and off and it is at that moment that noise starts, so only when there is a complete loop between both speakers.
Still, my thoughts are that all cathodes get connected together and probably the power supplies work with absolute grounds while the pre-amp works with floating ground. When a single DINAS is connected, everything is fine because wherever the regulation is being made, it all can follow the ground of a single PSU. But when both are connected the regulation cannot follow both absolute sources and things just fluctuate and maybe this is due to a beating between the switching frequencies of the 2 PSUs. This is just all but a supposition, I am not an expert in electronics.
I am a bit afraid of connecting both amplifier plates to the same PSU, because maybe then there will be a different kind of cross-talk between the amps as music is being played and they display impedance fluctuations that affect each other. But maybe I am just being paranoid and it would be totally fine. After all in a normal amp, and also in this amp, all channels have a common voltage input source…so maybe I am just being paranoid.