Aftermarket Stereo Problems (amp/sub)

[worked95]

When you say "mid-range" speakers, I am thinking that the head unit has actually filtered out the lower frequencies so that the md-range speakers only "see" frequencies between 150 hz and about 7000 hz. That means that your sub woofer amp is getting the wrong range of frequencies. What the sub woofer amp needs is 50 hz to 150 hz frequencies. A good head unit will filter out frequencies below 150 hz when feeding speakers that are not subs. Also a good head units provides the user the capability to set frequency filtering ranges.
You should investigate if the head unit is sending the full range of frequencies to the mid range speakers or filtering out "uneanted " frequencies.