Hearing distortion at low levels 6.5"

I just purchased a new-to-me 2008 Chevy Trailblazer. It's the short wheel base version and does not have the Bose option. When I purchased it, the previous owner had already installed some Kenwood 6.5" speakers in all 4 doors. After I purchased it, I found that the stock radio head unit was shot and acting up, so I purchased a Pioneer AVH-X4800BS to replace it. I discovered that the PO didn't even wire up the speakers correctly in terms of polarity, so I straightened the polarity out while I was at it. In short, there seems to be a huge hump in response at 100-200Hz, but anything below 100Hz just falls off a cliff. I've EQ-ed the hump down, so things sound pretty smooth now, but I just can't get any notable bass out of it without distortion setting in. I have the lows boosted ~6dB on the head unit, but it still sounds light on the low end and I can't turn it up past ~80dB without distortion. I've taken my flashlight and checked the cone movement and I don't believe it to be anywhere near xmax. It only looks to be moving about 0.5mm. I guess my question is: what the dilly??? The speakers are rated to 40Hz (not sure if I believe that though) and have a sensitivity of 90dB. If all the specs are correct, my 14 watt rms per channel head unit should give me much more thump than I'm getting once you take into account 4 drivers and cabin gain. Obviously, I'm not building a thump-thump machine, but I'm not joking when I say my '94 minivan had more bass presence and clarity with 4x 7-watt stock drivers. My F-250 with a JVC KD-R970BTS with JBL GX862's absolutely beats the snot out of this setup! Any thoughts on 1) why my bass falls off a cliff after 100Hz and 2) why distortion begins way sooner than it seemingly should?