How can this design condition be met in practice? Regulation would certainly work, but if not executed perfectly, may damage the sonic quality of the preamp. A choke based power supply would also work if the choke were terminated by a high wattage load resistor. A simple resistor into large valued capacitor power supply is also suitable, which is great, as it is the cheapest and easiest to fabricate.

My recommendation is go with the last approach, but use a raw power supply with a B+ of over 500v and drop this value down to 300v with two large resistors in series and at the midpoint wire in a series of 47v and 52v zener diodes that total 400v to ground.  This zener string, which is a terrible choice in AC terms, works well to set a fairly stable DC voltage; fortunately, the further filtering provided by the remaining resistor and capacitor RC network does much to eliminate the noise of the zener string.

A high voltage, low noise zener diode can be made out of a transistor, a voltage reference, and a few resistors. The LED serves as a cheap and quiet voltage reference in this circuit. The cathode-to-anode voltage drop of the LED will be between 1.7 to 2.3 volts. TIP 50 transistor will have to dissipate up to 3 watts and should heat-sinked. The transistor can be replaced with a high voltage MOSFET or a tube.

Cascoded shunt regulation  is used in the last circuit. The heart of the circuit is the TL431, a wonderful little device made by Texas Instruments that is both quiet and robust. Its cathode moves up and down in voltage to maintain a constant 2.49v at its reference input pin. A small cap, 1 nf, might have to be placed from this reference pin to the cathode of the TL431 to stabilize the circuit and the 1 µf might have to be increased for the same reason. The 12B4 can easily take the 390 or so volts from its cathode to its plate.