This is what I did to the car:
Pioneer DEH-P580MP Head Unit
Blaupunkt GTc652 (Front Door Components)
Kicker 055KS65 6-3/4" 2-Way Speakers (Sail Panels)
Blaupunkt GTx402 4" 2-way Speakers (Hatch)

With the stock head unit, it sounded good, had good bass. I put a after market head unit in, sounds good still but doesn't have the same amount of bass as it did with the stock head unit. Sounds really good, but no bass. I messed with the settings on the radio but it didnt help all that much, I was told the only way to fix this is to add a sub? Which looks like what I'll have to do.

 

Check if there's a high pass filter (HPF) engaged in the head unit. If so, turn it off.

 

the mid bass speakers u took out of your car were made for bass ...sort of.....not a true sub woofer...but it didnt have high frequencies goin through it....and it was a 2 ohm speaker.....u will not get that kind of bass with a coaxil speaker.......not to mention the speakers u have in there now are most likely 4 ohm speakers......do a search, there are mid bass speakers that are made to go into the sail panels.......the names escape me at this point, there are threads out their. good luck

 

You could also try adding door speakers. That may help fill out the sound a little more, although the real answer is indeed to add a true sub to the mix.

 

was this a monsoon or bose system?

if so, there's your problem. running a high level signal to the input channel of an amp will cause lots of problems and eventually kill the amp(s)

 

I don't think he added any amps, just changed out speakers and head unit.

But therein lies the problem as IIRC the speakers are all driven by separate small amps that are probably still in place, and they are also designed for 2 Ohm speakers So basically adding a Sub will cover the Bass end, but a complete re-wire may also be in order.

 

any passive crossovers that are part of the coaxial could also cause a cut in the bass too.

rewire FTW

 

You don't have any speakers in your system now, capable of producing low frequencies. You may improve midbass response by deadening doors and tweaking EQ on your HU. Also if crossovers has position to put tweets down few db's that would do it. But general solution - add a sub.

 

The car has a factory 10 speaker, but its neither the monsoon nor a Bose system, just what ever they used in 93 Firebirds. Not looking to rewire the system, I liked the stock system when I had it, but as it got older the speakers blew, some where lost during the interior swap, so I basically wanted some thing a little better then stock. I'll have to start looking in to subs, I think a single 10 will do just fine

 

If you want REALLY great sounding 10" sub for a small stealth enclosure - Image Dynamics IDQ. Can be found for ~$100 shipped on forums. I tried many, around 15 different subs in a stealth box (including 10w6v2, which is also great, but the value is poor), and IDQ so far the best.

 

My budget is around 150 before shipping for the amp and woofer, I'm looking at getting a Kicker CVR10 and a amp off fleabay, found some decent deals. Hard to see whats what, I have never been in to stereo systems, so far as brands go, hard to tell whats up.