You don’t need a sub or preamp out for the Vanguard,
I called crutchfield they have a little board you plug your rca sub cables into
then the other side get 4 small pieces of Awg 14 or 16 wire, Put bananas,or spades on one end the other end to the board ,bare wire they have wire clamp holders put a piece of Velcro on back
to hold it in place and your done ,it is actually more accurate coming from the speaker connections.
I called crutchfield they have a little board you plug your rca sub cables into
then the other side get 4 small pieces of Awg 14 or 16 wire, Put bananas,or spades on one end the other end to the board ,bare wire they have wire clamp holders put a piece of Velcro on back
to hold it in place and your done ,it is actually more accurate coming from the speaker connections.