That is dumb...a stereo shop has suggested that you give up your amplified speaker sound and go with the head units power???
Let me throw my 2 cents here.
Call Crutchfield and get the harness that will plug into your STOCK wiring and on the other end will plug into the PIONEER head unit.
There is ONLY one thing to take into consideration about your "Amplified System".
Since the PIONEER head unit delivers more output power than the STOCK head unit you don't want to turn the system "WIDE OPEN" since you could overdrive (over power) the stock system. Use it...just DON'T abuse it!!!
My PIONEER uses the complete STOCK AMPLIFIERS AND SPEAKERS THAT CAME IN THE XF WHEN I BOUGHT IT!!!
I like to "FEEL" my music and with the top down and running 145 MPH I want ZZ Top talking to me!!!