my friend told me that all my speakers in my range rover are indvually amplified, i want to run better speakers to the system so my mids keep up with the bass, so can i just add speakers onto the additional setup so i have my stock 12 speakers going and my new set of components working?? or is it simply one choice or the other?? also if i do keep the stock speakers in it, can i add a head unit w/o having to replace all the speakers??