Because stock head unit of montero has no RCA output(casa don't include it for some reason), i only used line converter to put an amp and to power and upgrade speakers, last time, i installed DLS COM07-06-R6A component sets in the front and some coax for the rear fill, and amplify it, the whole sets, including the installation cost up to 21k, wala ring sub, so i have to push the speakers to produce a low frequency down to its limit without distortion..
The problem always down on installation, for conversion like from high to low, installer faces problems like static so he must know tricks to manage that noise, so always consider who to install, in conversion also lost the front and rear fading, if these happens, the quality of sound decrease and out of line..