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Here how I look at it.

Hi.

You are correct for IDH tubes, particularly for phonostage, pure DC
to power the heaters from a battery is ideal. I concur no grounding is needed.

I did the same with my 6AH SAL cell powering the heaters of my IDH phonostage tubes, totally isolated & floated from the amp circuitry.
No hum & dead quiet pitch dark background.

But don't overlook the cathode-heater capacitive coupling, though in term of small pFs due to close spacing btween the cathode & the heater packed inside the tube body. This small inter-eletrode capacitance will allow noise & RFI to couple from the heater power supply into the signal path AND vice versa.

This will bring the need of some AC shunt filters. I installed in the
SLA cell O/P circuit inside my standalone heater charger/PS unit a
motor-run AC fan oil filter (7uF250AC) & a Philips 0.33uF220VAC film
box cap. across + & - O/P terminals. NO grounding whatsover.

Also across the I/P terminals of the battery PS in my phonostage I also installed 2x2uF100V film capacitors in parallel. Again no grounding whatsoever for the heaters.

Somehow I find it sounds better than without any shunt bypass caps
(which I built them on/off switchable) added to the battery terminals.

But for DHTs, as Dave has said since the filament is itself a signal path, I would add a CLC filter in between the battery & the filament circuit of the tubes. Then grounding is unavoidable.

c-J

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