Hissing is coming from your SUBS? JL subs especially?? They shouldn't even be able to do this.
Ok, first things first, goto your RF amp (they're pretty overpriced, but I got a good deal on one and have one too
and there should be a frequency cutoff that cuts off all frequencies above a certain point (like 25 hz).
Like I said, it's impossible that the JLs- or really any subs- can project high pitched hissing- but lets do this and then worry about the impossible later.
Any change? Try disconnecting the RCA leads to the amp- any change?
Are they even RCA leads? Or are they high level inputs? That might be the cause of the problem.
In any event, the hissing is most likely coming from your other speakers- does it relate to the position of the throttle? (IE, if you rev, does the buzz increase)
Another thing to look for is if either the head unit or the sub amp have bad grounds- try (if you can) grounding them to the same point on the car. This kills hiss like no other.
If you can't do that, try buying a Ground Loop RCA passthrough from radio shack.
But, chances are, it's probably just time for a new headunit that has RCA preouts if you're using high level inputs.