ack clipping!

You are not maxing the speakers...they are just playing the sound coming from the deck.  At that volume level, the stock deck is distorted.  The speakers can play much louder than that.  You can either replace the deck and add an amp, or use the existing deck and add an amp.  The choice lies in your wants and needs.  If the deck gives you everything you want, except for quality sound, you can keep it.  If it doesn't, and you would want to add a CD or something, replace it.  But the common factor is the addition of an amplifier.

When you say "2 cheap Rodamsters" are these subwoofers?  And you connected them to your factory stereo, right?  Did you leave the original rear main speakers connected too?  If so, then the main thing you are hearing is indeed clipping due to the fact that you are now completely overloading the factory stereo.  You have placed the Roadkill, err, masters in parallel with the original speakers and are now placing as little as 1 or 2 ohms load on the poor little amplifier in the factory head unit.

Stop.  Disconnect your cheap roadkills or you will fry your head unit in short order.  It is not designed for this kind of load and it will smoke.

If you want to run subwoofers, you will need to invest in a seperate amplifier to power them.

A person can whack his car audio gear out in short order by not understanding the principles of impedence, voltage and current.  That is, Ohm's law.  Read through the first few pages of this illustrative tutorial for a primer on these principles.  Be sure to read the page titled 'Installation Primer', too.

In short, an amplifier produces voltage, and requires a load of resistance to send current.  Speakers connected to an amp provide the resistance, in this case its called impedence.  If you put a high load of impedence on the amplifier, it sends out very little current.  You get very little sound and the amp is fine, not doing much work.  But if you put a low load on the amp, Ohm's law shows that the amp will respond by sending out a large amount of current, which is what fries its circuitry.  So if someone were to add speakers to existing ones, either in car or home, they will typically connect positive wires to positive and negative to negative to add the speaker.  That is called parallel wiring.  It reduces the impedence by half, and in most cases will kill the amp. 

The factory deck has a small amp in it which provides the power.  When an amplifier is added to the system, the deck is then a preamp.  The power would come from the amp you added, with the input signal coming from the deck.  The speakers' impedence rating and their number provide the resistance for the current to flow.

Bookmark that link I gave you, and refer to it often when you make your changes.  I'll bet you read through several of the posts on this forum and they didn't make much sense to you... but soon they will, and you'll get a lot of useful information by reading them.