Gonk - one thing I didn't mention was that when testing the Fathom as slaved, even with the volume all the way up, it wasn't "THAT" loud - loud but not overpowering, and overpowering is what I would've expected with the volume maxed out on the sub (and the 1070's channel calibration set to 0.) This was part of what set off a red flag to me, that the Fathom might not be getting a good signal.

My understanding of the Fathom is that the passthrough is/should_be transparent. However, my ears tell me something's up.

So with your suggestion, I could Y adapt right at the receiver, send one cable to the supernova and another to the JL. I'd use another Y at the supernova per the manufacturer recommendation. I like it. I think that's what I'll try.