Changing tube, does it use AC or DC filament heaters?

It uses DC. Also, filament current is designed for 5670 Standard, which is 0.35A @ 6.3V+/-10%. Do not use tubes of other ratings.

Also 5670 has a very unique pinout (RETMA code 8CJ) that is not shared by other tubes. Only 5670/2C51/WE396A tubes that share the RETMA 8CJ pinout, and are electrically equivalent, can be used. Any tube that is pin-compatible and electrically compatible with the 2C51/5670 will work. WE396A are often co-marked 2C51, they will most certainly work. Pin-out in the iFi products are all for 5670, so no adapters needed.

Please note, tube rolling or opening the unit will void the warranty.