Can an external preamp be connected to the return jack of an amp block and the guitar connected to the external preamp instead of the "guitar in" of the helix, and then that works like a real amp in that it bypasses the preamp of the amp block and only the power amp is emulated?
How about taking a preamp-only helix block and routing that to the effects loop, then running a cable from the preamp-only send into the "return" of a different complete amp block, thus bypassing the preamp of the complete amp block and using just its power amp?