Since you already have a 4 channel amp, why not add rear speakers and see where you stand from there? The only thing i have noticed is Rockford rates that amp only for 4 ohm loads per channel, it should still have enough to bring alot of sound from where you are with just front speakers ( if I read your post right). As far as a "dirty" signal, all the amplifier does is just that it amplifies whatever your source is putting into it. The object of having gain controls on a amplifier is to try and match up the input sensitivity of the amp to your source while it still has a "clean" signal. Almost every head unit or source has a clipping or distorted point on the volume scale, the hard part with your case is using your Ipod some of the actual songs will be recorded at different levels which makes it hard to come up with a fixed "max level" or clean signal from your head unit. I would add rear speakers to your setup and get a good local shop to help "tune" your system and see what happens. Just my to two cents, hope it helps.
2013 Cross Country, RJC exhaust, Lloyd's Filter, Timing wheel, PCV, Adjustable Rear lowering link, Goody's front lowers. NW New Mexico