Amplifier in my cars?

I have just purchased a Lexus nx 300h hybrid car

I love my music loud

I used an ipod

some music was low others were loud

i then converted the music using mp3gain

now the music is a pen drive and the quality is crap loud but crap

i want to instal an amplifier and change the speaakers but the garage advise not to as the radio is the gps etc

any savvies out there who can recomend something please as I am frustrated

okay, music can relax the mind of customers while traveling a long distance. We also implemented such facility in our radio cabs or taxi. According traveler's choice of music we keep their request. http://dawsoncab.com