Strange, but theoretically if you have the head dc supply voltage present but the head motor doesn't turn - then you would suspect the head motor.
But you have fitted new....
so, either new head unit is at fault (unlikely)
incorrect head ? is there more than one type for that model ?
or, you have a voltage present ( 310vdc ), however you are i suspect reading this voltage off load - i.e with no head connected.... i suspect if you measured this voltage on load (with head connected ) that the voltage will drop to zero indicating a fault on the pcb also.
In many cases with motorized head units it can be chicken and the egg diagnosis, and in many of them cases a failed head motor will also destroy it's drive pcb.
On these later vacs, the drive pcb's are a little more complicated than old dc24,25 models - dc40 onwards (erp versions especially) will not forgive a knackered head motor as often, and usually destroy the pcb drive.
Non erp versions are a crude rectifier, capacitor and a couple of resistors etc, and will forgive abuse.
I would in this case, again confirm voltage is there with no head connected, then connect the head and measure the voltage - it can involve stripping the head or ball assembly to get access to the wiring.