but reading on the subject of chokes i note that at lower currents the inductance can drop significantly. the choke is a 6h 200ma one but the phono runs at around 20ma so i suppose the inductance will end up negligable and the volts will shoot back up.
Not sure where you read that, but its wrong. The reason the voltage shoots up when the current draw is less than the current required for choke input, the critical current, is because the DC current in the choke is less than the AC current in the choke, so the low point of the AC current hits the zero current point, so current flow is not constant, so its not choke input. Nothing to do with the inductance of the choke varying.
What may be the source of the misinformation is that "swinging chokes" were used that were designed to have much higher inductance at low current, so at low current the system remains in choke input.
Simple rule of thumb to calculate the critical inductance, divide the input voltage by the inductance to give the current in ma. So with 250v (assuming you get 340v with cap input) and 6H, the minimum current is 42ma. So after the choke the voltage will be 225v, to pull 42ma a 5k resistor will do the job. But it will be dissapating 10w, so you may not see any improvement in heat output. It depends on how much current the phono stage draws, as you can subtract that from the 42ma. But remember that before that current is drawn (valves heat up) the current will be less than critical, and the voltage will be back to the 350v, so caps and everything else has to be happy with that startup voltage.
Also remember the current the choke has to pass is not just the DC current that is output from the supply, it is also shorting the AC to ground via the following cap, so it has to also handle the AC current given by the supplid voltage across its reactance at 100Hz
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.