I have a little Alpine amp that can run two speakers wired normally and run a bridged channel at the same time to the sub. I hooked it up this way but need a way to just send the low end to the sub. Is there an filter I can put in front of the sub? Also, any advice on gain settings? I just don't have any experience.

 

Do some reading on running amps in tri-mode. Check out www.bcae1.com and see #53 on the right. You need capacitors in series with the "full range" speakers as passive high pass filters (base blockers) and a coil in series with the subs as a passive low pass filter. You can get these passive tri-mode filters in one package, but I wouldn't. It would take way to long to get into why there are much better ways of doing this.
But I would really just pickup anther amp for the subs.
If funds are low run the subs off the amp and the other speakers off the HU. This is probable a better option then running the amp in tri-mode. Dose the amp have any x-overs?

 

The head unit only has one pair of RCA outputs. I had the components hooked up to the HU before and with the amp I can turn it up and there is no distortion. Funds are the issue, I have been trying to use what I have and get the best out of it. I will take a look at the links. The amp has a low pass which works across the all the speakers. Thanks.