You might want a "Y" cable to split the signal to your old receiver if your new receiver only has one RCA output for the sub. If your new receiver has multiple sub outputs, they usually carry the same signal, so then you could use a typical RCA pair cable.
You'll enjoy the feature of "bass management" that the 5.1 receiver brings: now you can block the subwoofer's range of frequencies from your main speakers, freeing them the effort of reproducing them. Or if you prefer them to keep doing so, at least now you can adjust the relative level of the sub far more easily.
Sources/processing: stack of stuff that if it isn't vintage now, it will be soon!
Amps: stacks and stacks of old iron
Main speakers: big DIYSG
Surrounds: Bose graveyard
Subs: a bunch