amp thump when car starts, hu off

Hi everyone,

Was hoping you could help with any ideas on this one. I've done some installs myself in the past and understand all the basics (plus have searched the forums already). Can't figure this out and need your help. Everything installed/built by me:

2008 altima sedan 2.5s (non-bose)
Alpine CDE-102 with KTP-445 inline (using kit harnesses but radio is grounded directly to tubular frame structure in-dash)
Stock speakers (for now)
2 eD 3.6 subs (each wired for 2 ohms, running in stereo with amp)
American Legacy la-360 (yeah, it was free so don't laugh)

OK, using 8 ga power wire (power down passenger side, rca's down driver's side) to connect this up. Ground is in back of the trunk on bare metal directly attached to sheetmetal/trunk lock bolts. Ground cable is also 8 ga and no longer than 2 ft.

Problem is the subs thump when I start the car. The HU is off but they thump a few seconds after the engine fires. If I turn on the HU, no thump. If I turn off the HU, they thump again. It's noticeable but not outrageously loud (I've heard really bad thumps before). I have no static, interference, ground loop whine or other issues. Sound quality is good throughout when playing any source.

I've already tried moving the amp ground (under rear seat, bare metal) with no change. I removed the RCA cables from amp and it still thumps. I remove the remote wire and it still thumps. Only thing left is the amp itself, right?

Before I get a real class-d monoblock amp (LOL), is there anything else I can look at? Thanks.

Update #1:
I wrote down that I removed the remote wire and it thumps....that is incorrect. I went out and tested this. When the remote wire is removed from the amp, no more thumps. So it's related to voltage on the remote wire.

Has anyone seen an Alpine power a remote wire (temporarily) when a car is started? When I start the car with the HU off, I get the thump but check the amp and it's already turned off. Seems like a momentary voltage runs down the remote wire (turning the amp on) but then disappears.

It's either the HU doing it or some magnetic field in the bundle the remote wire runs next to in the side trim. I also get a turn off thump when I turn the radio off (which points to the HU). Should I just consider dropping in a 12v SPDT relay at the amp to alleviate this?