adding amplifiers to factory headunit any better options?

Hey guys new to this forum and have a dilema.
first things first. Have had alot of prior expierence with car stereo but always with aftermarkey headunits as the starting point or building block per say...

I just bought a 2001 lexus IS300 and wife does not want the factory six disc removed from dash. I want to add @amps to the system. Fist wil be a kicker 4 channel to run all the mids and highs and will instal using the line level "speaker level inputs instead of "RCA's" also adding a 2 channel amp bridged to run a 10" solobaric but here is the dilema. I want to be able to constantly adjust the bass output or level.
so is there any component i can install that will allow me to adjust bass output from drivers seat like some of the higher end amps have a remote located knob. the amp I am using for the sub is a generic and may just have to step up but would like to hear from some others again I have never used an amp run on line level inputs.

Is there an adapter that can plug inline to the factory deck to allow rca connects for amp outputs,video etc. any help would be greatly appreciated. Thanks JIM C