Yeah, I like Steve's idea. Use pots to find were you need to be then replace them wit fixed resistors.

So you think the Sonys don't have any gain in the preamp circuit, yeah? Could be. Some of my vintage Pioneer and Sansui gear was like that, the pre outputs were really just after the selection switch. Volume, gain and tone were downstream. Too bad.

You kinda-sorta need a preamp to drive this rig. It would have input selection, gain, volume and any other gadgets. The output of that would go to the DCX. Or am I misunderstanding you signal flow?