I suspect the OP is having access difficulties. If I'm not mistaken
that unit has the amp built around in like a "box" and is fan cooled
through the middle. Ugh.

A shop I worked at previously actually took to replacing the whole
assembly, though I have no idea how they worked the cost of it into the
estimate. I think MCM sells those assemblies. I don't remember how much
they cost.

I've also noticed class D amps in subwoofers, and if that's what you
got this method won't work. You can tell by a pi filter at the output.
Class D amps lend themselves well to subwoofers because they can get
away with a lower chopping frequency. With their efficiency they are
singularly suited to the massive current needed to drive some woofers.

You mentioned bias, so it's not class D, consider yourself lucky, class
D amps are a whole different animal.

JURB