If you want to directly hook a passive subwoofer to the Pioneer head unit featuring Direct Sub Drive (check if your Pioneer unit have this feature, most of their models have it), here is a quick shot at the manual page describing how to wire it correctly. This page is from the MVH-160UI unit, but it is also the same for MVH-X370BT, MVH-170UI, MVH-170UBG, MVH-170UB:
When using a subwoofer of 70W (2 ohm), be sure to connect the subwoofer to the violet and violet/black leads of this unit. Do not connect anything to the green and green/black leads.
