Great! My suggestion: ( I do enjoy this stuff and have my share of time put in)
"Source B" the modern hidden-away unit is the MAIN UNIT. Outputs of which are wired to your amps and speakers, as normal.
"Source A" the period piece is an AUX INPUT to Source B.
If 'Source A" is indeed period it has either a stereo RCA output (maybe even 4 RCA outs), or speaker outs only.
If it only has "speaker outs", buy a Speaker-to-RCA adapter and you are set.
Your modern unit almost definitely has a stereo RCA input.
When you want the nostalgia, set your modern "source B" to the Aux input and enjoy your nostalgic experience. The RCA outputs on period head units will change the RCA out volume with the volume knob, so you could adjust the volume without grabbing for "source B" every 20 seconds. You'll just need to learn the relationship between the two volume knobs.
When you need all the extra goop, you're all set as "source B" has been wired to its full potential to your amps and speakers, and your Front/Rear/Sub situation is handled by the modern unit, with no fuss.
