So, if I understand you correctly, you take the positive left and right from the amp (Kicker Hideaway 8) combine them and attach to the Blue/Gray...And same for the two negatives?
I'm not that familiar with that sub. But it looks like it's just a powered sub with the amp built in. You can connect the sub channel to both the right and left inputs, but you only really need one. It shouldn't make a difference. It's a mono sub and you are feeding it from a single channel sub output. Connecting both channels to a single source shouldn't make a difference. Usually on powered subs the two channels are just combined internally anyway. If it were me I'd just put the sub channel into the left channel input.