How does the drive current fuzzyness translate into sound?

Wouldn't their be a fair old smoothing of speed due to the innertia of the deck compared to fast adjustments of drive current?

Must also depend on how the servo type thing works - are different currents used to vary the turning force or is it down to timing of the currents to the coils?

If the servo opperates via changing the amount of current to each coil (i.e. turning force at each point) then the timing of that delivery would stay constant and only determine absolute speed accuracy and the speed slider circuit quality might have a much much higher order of magnitude of effect (or removing it might be better). It might not to be to do with anything in the servo loop.

That 4MHz seems familiar as if it is the same value used in the Pink Triangle Too PSU. The Too PSU uses logic to recreate an AC drive to the same Premotec motor as used in the Linn.