That bass boost at 75% is a good way to send a clipped signal to the subs. Excessive clipping turns into heat in the woofer's motor assembly and will kill it just as bad as direct DC current will.and my bass EQ on the amp is set at about 3/4 way
Sadly, most do NOT hear distortion until it reaches close to 10% in the sub bass region whereas others don't notice it until it gets closer to 30%. Regardless, just because there is no audible distortion, your amplifier could still be sending a clipped signal to your subwoofers.it does not sound distorted at all, it sounds great but that smell continues to come. from every sub and even on the sealed enclosuer sony.
Going further, I believe your pre-fab enclosure IS NOT the right size for ANY 12" subwoofer ported, hence the reason your subs hit the "high notes" and struggle on the low notes. I could be wrong, but that enclosure just looked way too tiny from the photo you posted. Generally a modern day single 12 will need anywhere from 1.5 to 2.5 cubic feet ported with the port tuned between 30 and 35 Hz. If I had to guess, your Fierce Audio enclosure is probably 1 cubic foot tuned in the 40 to 50 Hz region. Truthfully, you would be better off with a 8" or a 10" sub if you must live with a 1 cubic foot ported enclosure.
Until you address your enclosure issue, I am afraid you will not have the deep bass that you crave.