hitachi

new to the ls400 and wanted a breakdown on the cars audio system
mainly wanted to know if there was an amp other than the subwoofer amp? like a speaker amp? or are the speakers and sub powered by 1 amp?
and if there is a separate amp ,should i replace that speaker amp if i install aftermarket speakers?
see ive installed aftermarket speakers on my rear doors but they still sound a little distorted, even at a low volume? would getting a new head unit fix that issue?

another thing i wanna ask about is front door speakers . i cant find any 4" component speakers that ship to america,

and finally if anyone can explain the details of what i might not know when installing an aftermarket head unit regarding weather or not i need to any to replace factory wires.. any heads up

thankyou