-with indirectly heated tubes and a power amplifiers, DC supplies for the heaters (both driver and output tubes) are often not necessity, especially with PP amps
-preamps, especially low-level signal types (mics, RIAA) usually need DC-powered heaters
-filaments in DHT-based power amps (power tubes) can be AC operated, but sometimes DC in needed, in SET low power amp + high efficiency speakers cases. Some people claims that AC sounds better, especially in comparison with simple/bad DC fillaments supply
-there`re several types of DC supplies - from simple, pasive types (CLC, CRC...) to the regulated ones. With IDHTs, simple series voltage regulators (transistors or ICs) are "good enough". But, with DHTs (where filament = cathode = signal path), probably more "refined" versions are in order (CCS, shunt regulation...)
-see, for example, some heaters/filaments and supplies article by Mark Kelly:

• http://www.members.iinet.net.au/~quiddity/audio.html