Ok I found some of the problem...
Need to turn off the HU amplifier if you are going to get anything from the preouts. So I do this and read 0.7 VAC out the RCAs of my HU. Ok great.
Now that ~0.7 V goes through to the subwoofer speaker outs on my amp with 0 gain.
Now I turn everything down (HU, aux input from FreqGen on iPhone, amp gain)... wire up my sub.
Turn it on, slowly increase volume, and I got nothing. Turned up the HU volume, iPhone volume to full, made sure I was still on Aux input. Gain on amp is 0 at this time. Slowly turn up amp while reading with DMM... Go all the way to 10 VAC, nothing.
I beep checked my RCAs, beep checked the whole signal path all the way to the dual voice coils. Everything is cool.
Now I got a problem.. lol
edit: also flipped between 2 4 and 6 ch settings on amp. Also made sure LPF wasn't cutting out the 50 Hz signal.