Ok I actually bothered clicking on your link... sorry it appears you have tried most things and my understanding it is not mains frequency hum.
I have experienced weird noise from various heater issues and after elevating heaters above cathode potential many never appear, so that has become as essential as grid stoppers to me.
I have not used CCS on my 6AS7 pre, just a resistor, and I cant recall it being noisy. At the moment there is a 6BX7 in there b/c I need a little more gain. Other than low gain the 6AS7 pre sounded great.
My power transformer gets very hot after about an hour, and it's a bit big and bulky. Since the amp is pretty successful otherwise I just reclassified it from a "junk box build" to a "project" and ordered a custom power transformer from a local company 🙂 It will be a smaller and more effective design than the Marshall-type transformer, with more current capacity, and I also specified a 5V winding so I won't need a separate filament transformer for the rectifier.
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When I'm looking at the plate curves for the 6AS7 I realize that my bias must be way off. The green point on the graph is my voltage/current at the moment, but I only have -18V bias. Please correct me if I'm wrong here, but If I'm aiming for say 100V/50-55mA at idle (red/blue lines) I should get closer to -40V, so I should increase the 330R cathode resistor a fair bit. I'm thinking something like 720R should get me a lot closer to a good operating point?
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720R cathode resistors resulted in -36V bias, but higher plate voltage, so I'm now at 125V/50mA. I figure somewhere around 90-92V on the plates and 55mA should hit the bias spot. Should I reduce B+ to get there, and then fine tune the CCS? Or am I overthinking this?