Mac C8/C8/S in the late fifties~ early sixties did the same-with the resultant @ 8 volts DC filament(heaters after voltage rectification w/nearly doubling the 6.3 volt AC with the conversion to DC heaters for all six 12AX7/7025 tubes per preamp when fed from the octal cable connection from a Mac 30, 40, 60, etc. stand alone tube amps connected via the octal umbilical cord..
Or a stand alone PS for said preamps, as there was a MAC optional stand alone PS to provide the necessary power filament/HT for those nice sounding preamps should they be used without Mac amps providing the necessary power for use with other amplifiers with non compatible octal connection, if used..
For the time frame, the lowered Filament voltage, and being DC did result in overall lower system noise floor due to Mac`s configuration, for the engineering mindset at the time..
I owned and used said preamps during the almost mid seventies until I sold them to my B-IL in the mid eighties after building a stand alone very well regulated stand alone SS rectified HT/12 volt Filament PS for their use for him prior to sale..
He refuses to sell them back to me when I have offered generous return/profit on his purchase....