D.C power supplies are advisable. In D.C Z= R as frequency f= zero Xc or Xl would not reduced the current of heat production could be optimized as W=(I)(I)R....

I would advise to use a grid frequency step down transformer since it is very reliable. To adjust the power, a tab changer can be used. The problem with a DC power supply is, that it either needs large capacitors or generates a lot of harmonic currents into the grid. Also the output current is high and the efficiency would suffer with DC.

DCDC is better, as the Switching frequency related, a DCDC can be up to 100KHz or more , so small size is OK. But the thyristor only ~ 10KHz, that means system size is big.

I havent tried DC but I use a AC stepdown transformator. Input 220V 50 Hz output, 1,3V 500Amp