Pre Amp Tube heaters question

Hi,
maybe this could be a silly question but I'm not an expert...
I want to know the differences (if there are any and the drawbacks) if we use DC instead of AC to the vacuum heater (for pre amp tube) in A15MK1 but also in other mad amp amplifier.
From what I know DC should prevent hum, so why we use AC in madamp kits (and maybe risk to have annoying hum)?
Is this just a cost saving reason (Ok if we use DC we have to rectify the output of the power supply) or there are some disadvantages?
What about the power amp tube? is it possible to use DC for the heaters too?
Thanks for your help.
Krusty