A portable cd player plugged into the amp resulted in no noise, so I guess
the amp is fine. Once I get time this weekend, I will play with the head
unit ground. Do I have to ground the HU at the same point as the amp, or
will a good ground up front suffice?

No, just aim for a good ground. As you may have heard before, eliminating
noise is more of an art than a science, so I wish you luck. Some cars and
some equipment is more difficult than others. That's what I meant when I
suggested you had a professional look at it. Not that you're incompetent or
that you couldn't figure it out. But rather, it can be a real pain in the
ass to finally figure out and they don't charge extra to do it.

couple random questions:
Do the power lines to the HU sometimes give noise like a bad ground would?

No.

Do power filters, ground loop isolators, noise supressors, etc work? How?

They can. They work by isolating grounds.