Running both crossovers is no big deal. I often set the amp to 100hz or so and then the driver can vary the crossover in the deck to what they like but they'll never get full range. This can be super handy on the high pass too as the driver will never be able to send full range (ie bass) to the front speakers

Check the position of the 2ch/4ch switch. Although you have 4 RCAs feeding it, if you set it to 2ch it will ignore the 3/4 set and play everything off of the 1/2 set - which if that has 80hz HP at the deck means that your amp sub channels are getting no bass signal..

 

With more consistent voltage it should give me power correct?

The Birth sheet shows:

123 watts rms x 2 @ ohm. 14.4volts
320 watts rms x 1 @ 4ohm. 14.4volts

So I was thinking the big 3 and maybe a cap would help me out.

Am I correct on this?