"You can tune the eq to your liking" does that mean adjustments to bass mid treb in the radio do work or do you mean adjusting with an aftermarket amp.
Aftermarket. The head unit EQ would no longer function, nor would the balance and fade, if I'm not mistaken. The stock setup is more convenient, but a audiophile would be much happier with the ability to tune the signal however they want with an aftermarket EQ.
I'm a little new to things? Would you be able to explain some of the terms and instruments?
The head unit is the part in the dash. That sends the digital sound signal to the factory amp which converts the digital signal to an analog signal and amplifies it to run all the speakers. You cannot replace the head unit with any amount of ease, if at all.
The factory system drops bass (to the subs) as the volume goes up to protect the speakers. I'm not sure if the head unit or amp does this, so I don't know if Forscan can correct it.
An aftermarket sound processor pulls the signal from one or more speakers, and processes it in various ways depending on what processor it is. Mine is a simple 2 channel sound processor that currently ties into one of the stock subs to get the sound signal, so it gets the factory bass signal that doesn't scale well past a certain volume, and only gets low frequency bass. My sound processor is supposed to counter the bass drop-off by increasing bass exponentially as volume goes up, but since it only gets the volume level of the bass signal, it doesn't know the volume keeps going up for the other speakers. My Sub doesn't get much louder past volume 18.
The door speakers get a fuller range signal which would work much better for my aftermarket amp/sub, but those speakers also provide the noise cancelation and "fake" engine sound. If I tied my amp/sub into those, I would have a 2kw subwoofer producing fake engine sounds, and I definitely don't want that. That's why I would need to use Forscan to get a flat signal without noise cancelation. Doing that would allow me to tap the door speakers for my amp/sub so the volume works correctly, but then I would have to go aftermarket for my EQ if needed.
Long story short, I don't think there really is a cheap or easy option if you're looking for anything more than a little bass boost... Thus is the case with most new "premium" sound systems anyway.