Your low pass is set way too high.
But that's not the real problem.
The real problem is that you are saying you connected the RCA cables to the amp "output" side of the amp. The verbiage used on equipment regards signal flow.
Keep It Super Simple: If you want the signal to flow IN to the amp from your source, then you connect the RCA cable to the jacks labeled INput.
The OUTput RCA jacks can then be used to send that same signal on to another amp if you are stacking amps.
"I plugged a short RCA cable into the radio and into the input side of the knob and plugged in the RCA cables that lead to the amp into the output side" - as in the output side of the BASS knob, not the amp.
He's referring to the BASS control knob so I think he's good on that end. He's using the RCAs that feed a working JL amp to use a bonified source, when plugged into the Hifonics amps they don't produce any sound, they are on but have no output. Filters are set to make sure something the subs play, gets through. The OP has already confirmed subs work when tested with the JL amp.
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