Negative filtered power supplies, how they work ??

I am working on a tube amp that uses a negative filtered power supply..It was designed to use two 500 ohm field coil speakers...I would like to use it with modern speakers but finding what to use in the place of the field coils has turned into quite a problem..I could convert it to a more standard power supply but other issues in the biasing system are effected....I have put chokes in place of the coils , but have no idea of what value chokes to use , so here I am with this problem....I would love to hear from someone with experience in this problem.....Thanks Will