Well I've been playing with the values for SPWM frequency and resolution and found the highest setting for the 12F675 using internal osc was around
Code:
DEFINE SPWM_FREQ  200                ; SPWM Frequency
DEFINE SPWM_RES   256               ; SPWM Resolution
Its not possible to up the resolution without using an external OSC. However I intend to use a different PIC and may opt to use it at is max xtal rating, especially if I use the one PIC to provide all 4 channels. Maybe the higher resolution will reduce the grumbling in the armature... mind you if this was on old 0-6-0 shunter it would look and sound just right

One thing I did notice is that when using the PWM version the power tranny runs cold, where with the close loop version it warmed up (slightly)

Anyway, I'm having fun learning