Looks like what happened the author of the Install pdf:

I settled on the Kenwood KMR-D562BT. I purchased from Crutchfield because after the freebies, the price seemed similar to others. I had to call them to get the free stuff as this is a marine HU and their system doesn't auto include as a result. Advisor suggested that i use Metra 70-1786 and a Scosche SLC4 Line Output Converter to connect to the amp. He said the volume would be too low without the Line converter

Looks like sometimes using the pre-outs isn't always the best decision.