You need to find out what's going on before you start replacing amps or you may just end up with new amps that are still not as loud on the left side or you may accidentally fix a simple problem while replacing a whole bunch of stuff that wasn't necessary to replace.
Is this happening with all sources? Radio, CD, mp3/iPod/aux (if applicable)
It's possible that one of the pre-amp cables has come loose at the back of the headunit - that'd be the first thing I'd check, after looking through the built-in EQ, balance and DSP in the head unit's menus and making sure nothing goofy is set in there, like the "listening position" as Kilgore suggests.
On the left side of the car, is there sound coming from the woofer in the door and the tweeter pod and the rear deck? Is one of those out completely or are they all playing at reduced volume compared to the right side?
You may have a blown speaker, a bad connection, bad wiring or a bad amp - you'll need to do some troubleshooting to narrow it down.