Prepping to install a JL 300/4 amp. It will power a pair of components on the front channels and a sub off the bridged rear channels.

The JL manual says a bridged channel requires a mono input achieved by "using a Y-adaptor to split a mono signal into both left and right RCA inputs."

I have a Alpine 9851 HU with a sub RCA pre-out (2 channels).
Isn't this Sub pre-out on the head unit two channels of the same mono signal?

If so, shouldn't I just run a pair of RCAs from the Sub pre-out to the amp rear channel input?

 

The sub out on your unit will be a mono channel. Just go stright from your unit into the JL amp.

I have the same amp and have done the same thing.