When I replaced the speakers I also replaced the HU. I believe as long as you have a good quality amp and speakers you will get an great improvement over the stock "paper" speakers. Just get a line our converter and plug your amp RCA's into those.
Excuse my inexperience but i think the local shop told me i needed two LOC to keep the fade control right? Trying to hook up a 4chnl amp to new front and back door speakers. Would you just tap into the wiring at the closest rear speaker?