installed a new HU and now I get no bass

Hi everyone. Was wondering if I could get some help for my problem. I have a Pontiac Trans am 1993, with the stock everything. The stock radio was only cassette and I wanted to play cds so I got a new Pioneer DEH-P4600.

So I have it installed at Best Buy(free installation) but when I get home the sound is all screwed up. There was no bass coming from the speakers and the sound quality was crap.. It sounded like my friends radio which is just 2 speakers.

I brought it back to best buy and had one of the guys look at it...He messed around with the settings on the HU for a while and came back saying that the old stock radio that was taken out had a built in amp. So because I replaced it with the new pioneer there is no amp powering the speakers. He suggested to get a  cheap 2 channel amp so that I could get the sound quality back to when it was stock. My sister had a sony xplod 480 2 channel amp that she doesn't use so i bought it off her and came back to best buy...There was a different guy there that said the 2 channel amp could only power 2 speakers and that much power on 2 stock speakers would blow them. He said he never heard about an amp being built into the stock head unit. He also said that the 50x4 watt on the pioneer should be enough to power the stock speakers just fine. He also said installing the amp would be $140.

So my question is....What should I do?... Honestly all I want to to be able to play cds. I know nothing about amps or speakers or anything to do with car audio. How do I bring up the sound quality to the way it was before...I think I've been getting misleading information from everyone I talk to.

Any help would be greatly appreciated

I don't see the why I should get new speakers...The stock ones sounded perfectly fine with the old HU.

I'm wondering if the pioneer just isn't compatible with the stock speakers. Or is it possible the pioneer is defected or the install was bad.

thanks ravendarat. I'll be heading back there tomorrow morning to have them check the wires. But just in case for some reason they refuse to check the system(or god forbid ask me to pay them...heads will roll) what would be the correct way to check it myself.

Thanks again for all the help.

Ok I wasn't able to go to best buy today so I checked the radio out myself...I found out the wire colors for the speakers from here....

https://www.the12volt.com/installbay/detail.asp?info=alarm&year=1993&make=Pontiac&model=Firebird&ID=18766&type=Stereo

And using the color chart that came with the radio I found out all the speakers wires are connected to the right ones... so they are not out of phase I guess nor is the wire adapter connected wrong. Also I did a little experiment...I disconnected the pioneer unit and reconnected my old stock radio....And the music sounded great...great bass great everything. When I changed it back to the pioneer the bass is gone. I've been messing around with the settings but I still get nothing.. no bass at all.

There's only 2 things that I can think of.

1. The pioneer is defective.

2. The internal amp on the stock radio is more powerful(way more) than the internal amplifier on the pioneer.

I was wondering if anyone knew the specs on the stock radio for a 93 Pontiac Firebird/TransAM. I believe it's a monsoon sound system.

Thanks again