WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
This may be a dumb question, but if I'm using a choke that's rated at 150mA at 600VDC, can I uprate the amperage if the voltage I'm using on the power supply is significantly less, say 400VDC?
No, the current rating of a choke is normally determined by the point (current) that causes the choke to saturate beyond some percent (maybe, 80) of nominal. I^2R also needs to be considered.