intermintent xtant amp problem repair?

I've got a Xtant 6.1 powering one JL 10.

The amp has developed an intermintent problem.  The sub will pop, cut out, or thump if you jostle the amp around.  It typically happens when you smack the amp right above the section with the gain controls and rca inputs.

I opened the amp up to check for anything obvious (loose components, broken solder joints, etc) but couldn't find anything.

I resoldered the joints for all the pots (gain, xover, bass boost, and subsonic filter), as well as the rca posts, but that didn't help at all.

What should I try next?  The amp works fine when it's working, and seems to be directly related to a loose or poor-contact component.

On a side note, I did find a really cool feature inside the amp on the board near the preamp stage.  4 surface mounted LED's that are like a vu meter.  They show when you start clipping the input.  Not sure why they're buried inside.  It's a useful feature that could have been incorperated into the casing.  You'll never see them without pulling the cover and voiding your warranty...

Just remembered -

Some times when you smack the amp, the sub fully extends as if it's receiving pure DC.  Smack it again and it goes back to normal.

It's not under warranty.

I know it's broke, I'm looking for ideas on what to check so I can repair it.