I did a little more messing around and have found that the bass is at its maximum volume at 3 full bars of volume. From there on out, the bass level actually remains level regardless of where the volume is turned, until you get to the final last bar, then the bass is cutoff. In other words, the volume control does not influence bass output that much, it seems to stay the same basic volume at low, and high volume, there are obviously some exceptions on volume levels (very veyr low volume, or very very high volume.) It looks like I was wrong in my original assumption that it was the bass frequencies that were cut at higher volumes, it actually is not. However, what I did find, is that it is the HIGHER frequencies are actually boosted excessively at higher volumes. It just seems that bass is cut when you're listening to these systems, but it'* actually remaining level. The volume of high frequencies becomes much higher, sort of "drowning out" the bass. I found this out by leaving off the amps for my door speakers, and having ONLY the subwoofer play. I found that bass like I said above is at its loudest at 3 bars, then stays level from any volume higher than that.
I also found out that the peak frequency for the "bass" range on the headunit EQ is 50 HZ, the peak on Mids is 635 HZ and the high frequencies peak frequency is 12K. These are the peaks for when the "bass, mid, or high" settings are increased or decreased. I believe this is why the highs may sound so shrill at higher volumes. 12k is a apretty harsh frequency to begin with, so is 8K, especially when you're boosting those specific frequencies more than the other surrounding frequencies. With that said, I'm not sure how to get much better sound out of the stock unit because of the treble boost. You'd have to leave the volume at a constant lower level, then use something else to control the volume (such as the JL Audio Clean Sweep.) Other than that, you'll just have to deal with the boost issues. What I did was to set my gains high enough that I don't really want, or need, to set the volume much higher than 3 full bars. (and it'* still very loud.. I can't even stand the higher volume levels they're so loud.) From this, I get good, clean sound, without the crazy boosted frequencies. I seem to get the best "sound quality," and accurate sound at the 3-full-bar level on the headunit.
BTW, I have the EQ on my stock deck set to -4 Bass, -2 Mids, and +1 on the highs. I don't get any distortion regardless of volume, and it sounds pretty good, minus the shrill highs at excessively loud volumes (which I dont' listen to anyways.) If you have a subwoofer, just set your subwoofer amp to have a slight bass boost to make up for the -4 bass setting on the headunit. THis way, your subwoofer will be playing the bass notes, and not your door/rear speakers... Most cannot handle 50HZ of frequency anyways, especially the fronts in a stock configuration/stock speaker mount.