Click on this link then click the TA 4650 download link for the service manual. Pages 2,3 & 4 have the specs and cicuit block-diagram. There is only one volume control and it is in the pre-amp section.

I think it is worthwhile to proceed with purchasing one of the crossovers to place directly in the pre-main loop.
If it is noisy I can boost the input to the crossover with a matching-amplifier or matching-transformer and then attenuate the crossover output passively. Am I correct?

Marshall.