Have your tried running an extra earth lead from your amp earth point to the same earth point at your headunit? This should fix the popping noise.

Shop around for amps, I've had a load in my time - I wouldn't match the rest of your system with an infinity amp, I don't rate them, especially for the price!

The best amp pound for pound I have had and still own is a... Lanzar!
Not much street cred I know but loads of power for the money and lots of features too, including remote bass gains! Take a look at the Lanzar Vibe438, hard to find in the UK but very different to your Les Smith / Halfords stuff I wouldn't touch with a barge pole!
:banana: