Sc400 '93 Nakamichi System. Super Noob At Car Audio.

[Aggelos]

I came to this forum by Googling all I could about the Nakamichi system in my SC400. I recently found out that it has Polk speakers in the front and back and an 8-inch Polk sub which I believe is this one (http://www.amazon.com/Polk-Audio-db840DVC-8-Inch-Subwoofer/dp/B0016GV1UU)

So I went out and bought a new radio, (http://www.bestbuy.com/site/Pioneer+-+50W+x+4+MOSFET+Apple%AE+iPod%AE-Ready+In-Dash+CD+Deck/6261746.p?id=1218725174656&skuId=6261746&cmp=RMX&ky=1whxm51gbOgORrn6M8JOUlLjDtR0yY7Rx) from the local car audio place where I live. I had them install it because I know nothing about car audio and didn't want to end up !Removed! everything up. But now I've noticed that I'm actually not even getting the quality or loudness that I used to get from the stock Nakamichi headunit. Could it be that I need a new amp or that they didn't wire things correctly? I know they said I would probably need a new amp cause the stock wouldn't have enough power after they installed the headunit and that they would have to do an amp bypass. I'm not experienced in car audio at all, but I can tell you a whole bunch about PC hardware, software, and networking. I'm hoping to get a word from the wise here or at least find some helpful post by continuing my search on this forum.