Download Power Amp Equalizer. It has a 32 band EQ curve and variable input gain from the DAC in your phone. Set the input gain to like +5 and boost 30-40hz +3db for the low end bump similar to a ported box (still won't be as loud as one tho)
Another thing you can try is try moving the box further and closer away to the loading wall that is firing towards. My SQL12 in a sealed box got much louder when I put moved it about 3-4 inches away from my hatch.
If you are hurting for space and can't fit a ported box, you could try using a passive radiator to get low end extension without sacrificing too much space. The downside is that passive radiators usually have more distortion but it's likely you will not care, let alone notice unless you measure the distortion with a meter. Downside is building a new box
Could be voltage drop. Sealed boxes have more excursion over the entire bandwidth unlike ported boxes and the amp has to use more power to control the cone (keep in mind that it takes power to throw the cone, but it takes even more power to make it come to a stand still especially on a sealed box cause of the air spring). When I switched that same subwoofer over to a 1.75 cubed ported box, the voltage became much more stable and I could be more liberal with tbe gain, even with the +5 input from Power Amp Equalizer. My second subwoofer I had was a Comp R 12 inch in their prefab ported box and it had pretty decent low end excursion, however I didn't care for how little port area it had. Maybe like a 4x2 inch mouth
Hopefully you can figure it out man. Nothing is more disappointing to me than getting a nice setup I paid good money for only for it to deliver well below expectations