So If I get mercmans harness to use the balanced level line input signal to the JL amp, can you use the pre-outs on the JL to power another subwoofer amp?

Do the old 300/4 JL's accept balanced level input?

I'd like to use a 300/4 or similar for our 2005 non nav TSX system, and then run a small 500 watt mono sub amp from the pre-out channel output of the 300/4.

or would i need to use another balanced level amp like the JL 500/1.

The system for the TSX would likely consist of:

Infinity perfect 6.5 front components

6.5 infinity kappa coaxial in rear doors

remove rear 6x9s.

JL 300/4 for front/rear and a small D class 500 watt mono amp.

12" digital design 3500 series sub in the trunk.