Originally Posted by wpoll

A car fitted with the factory DSP amp has all the speakers wired to that DSP amp (in the trunk/boot), so to fit an after-market system either requires the continued use of the factory DSP amp (the head units feeds into the DSP amp) OR you can re-wire all of the speakers to by-pass the factory DSP amp (a LOT of work!).

Most folk seem to use the factory DSP amp when doing this upgrade - hence all the talk about the Resler interface and the iBus app.

As you mention, many android units also sport a DSP (Digital Signal Processing) stage. To simply things, you can think of DSP as a fancy equalizer and having two equalizers in the system isn't really needed but also isn't really a problem either.