I thought it may be that the LFE channel was out on my receiver, but after trying the sub on another receiver, I see that the sub is the problem.
Hooked up, it just gets a very faint bass signal. When I turn it up on the sub and the receiver I get a little more sound and a "pop" of bass on occasion.
Any ideas?
I shot an email to hsu tech support hoping they can help.

Brady, the problem is apparently in the sub amp, and the only suggestion I'll offer is to connect with speaker wire to the speaker level inputs to see if the problem is in the line level input.

Just talked to Pete at HSU. The problem does indeed look like the amp.(tyvm John)
On a side note. I AM SO IMPRESSED with HSU customer service.
Pete gave me detailed instructions on everything I need to do to get this sub back to life and did it all very pleasantly at that.
Kinda reminds me of the times I've talked to Axiom customer service.

That is good to hear. My Hsu VTF3-MKII has been going strong for 5+ years now.

This is one of the few times I have heard of a HSU sub having problems, good to know their CS is as good as Axiom's.