The pre-amps output to the headphone amp will (I assume) come from the pre-amps tape out. If so the signal will bypass the tube amp stage in the pre-amp so there will be no tube redundancy. If you are taking the signal from one of the pre-amps main outputs you will not only have tube redundancy but you will have attenuator redundance which is not optimum by a considerable margin.
If you could afford it you might consider a headphone system with a seperate CDP or a CDP with the signal being split and shared by the headphone amp and the pre-amp. The first is optimum.
Tubes & 650's are a great match.