If I could give more likes I would.
Those all sound like great advice. I am thinking that in my case it really is the operation principle of the tube pre-amp and its underlying electronics coupled with 2 distinct switching PSUs. I wish I had another AC/DC PSU lying around with the right plug and between 15 to 22 V just to test different combos.
I still am not sure that its a faulty PSU, as each PSU works fine with no perceivable noise when just one being used. But it can also of course be, I am not ruling that out.
In the meanwhile I ordered the 180W PSU because it was 60% off and I guess ill give it a try with the 2 pigtails, plus some ferrite beads on the ends.
But I will still look into all that you said.