Hmmm....all the 4 channels I've used had a single pair of speaker level inputs that by default feed all 4 channels, thus preventing the use of the front RCA inputs at the same time. Does the JBL have a switch on it to select different input types for different amp channels?

Interesting option......

 

The OP didn't actually post the what model the JBL 4 channel is. I browsed all the current models and none of them actually have a high level input. I have the last generation GTO 4 channel and it has both hi/low level inputs. So if it is a newer model amp, then you will for sure need a LOC in order to wire everything as I had suggested.

To BRG of all the 4 channels I've owned, they have always required a front and rear set of rcas to run all channels.

 

The JBL amp is the GT5-A604... so I do not think it has a high level input.
I would need a LOC.
Not just have to decide whether or not it is important to have sub control on the HU....

 

It's important to have it, though I find it more convenient to have a wired one, as opposed to having to go into a bunch of different menu screens in order to access it.

Yeah, your amp does not have a high level input, I'm sure there are tons of relatively inexpensive LOC's out there

 

There certainly are many LOC's.. I am now trying to look for a good one as I have read that sub par ones will effect SQ esp on mid's and high's (true / false???)

 

Just use the one set of RCAs for the 4 channel. LOC's are known to induce noise.

 

This is the easiest way, and the cheapest way (no LOC and only 1 set of RCAs)