It sounds to me like the problem isn't related to the Axiom speakers, but to how you have your system set up.
Yes, there can be a big variance in channels, and this is your cable company's fault. I know that with my Dish Network, the HD channels' volume level is always about 5db quieter than the SD feeds. That has nothing to do with your Axioms, your TV, nor your receiver. It's out of your hands.
Anyway, back to your problem...
First make sure that the center is hooked up properly. Make sure you don't have a polarity problem.
Try turning that Audissey stuff off. Calibrate your speakers with an SPL meter using the instructions in the 3803's manual (I'm guessing that they'll be in there). It will involve something like setting the volume to 0db and matching the output of white noise levels of all of the speakers. This is to make sure that the Audissey EQ isn't messing something up, as I've heard of this happening around here.
Also be sure that you don't have some sort of source-specific level control interfering. I know that my receiver can store different channel levels depending on the source. With a few mis-matched settings, it's easy to get it messed up and really affect the sound quality. Don't ask how I know this.
There are probably adjustable channel levels on the DVD player too, but they should only be active if you're using the analog outs. Since you're using the digital HDMI connection and using your 3803 as the decoder (as you should be), I don't think you'd have a problem there. Do double check the settings in the DVD player to make sure it isn't doing some sort of dynamic range compression or other tricks to play with the sound.
