I think the only step I have left is a filter that connects inline with the alternator, but I'm wondering if I missed something. The Crutchfield chart suggests I need a power filter, as do my own tests

Powering the amp from wall power via an extension cord from my house, but feeding though the same RCAs in the dash to the amp (with the amp turned on by shorting ignition sense to amp sense since the ISO connector isn't plugged into the unit) I get no noise. This as well as the Crutchfield tests absolves the amp; It also absolves the RCAs and the speaker wiring.

The Empeg is grounded only to the vehicle chassis at the same spot the amp is, with 6 gauge stranded copper. I also tried using one side of a jumper cable to ground the Empeg chassis directly to battery ground (as the only ground) with no effect, just to be sure.

An American International S10A power filter connected inline on the constant power and grounded does nothing for the noise. For the heck of it I also tried filtering ignition sense, again with no effect.

This implies one of:
-it's insufficient to take care of the noise
-the wiring on the sled itself is in some way hosing me, since the wall power test didn't use the sled wiring.

Is there something obvious short of alternator inline filtering I can try? Battery electrolyte levels are fine, and I've found nothing else which looks plausible.