this happens nearly everyday. make your box smaller and keep the tune. try 1.5 tuned to 32. your gonna loose alot of lowend but its going to sound much better uptop. make hte port 3 inches longer and add another port space 2.25 inces from the on on the opposite wall make it 12 inches long that will take it down to about 1.6 with a similar tune..
I've built 3 ported boxes for this pioneer back then. 1.2 cubic feet tuned to 35, 1.6 cubic feet tuned to 40, 2 cubic feet tuned to 30 hz and a sealed 1 cubic foot box. Neither of them had any performance up top. There's a steep roll off after 50hz with every box even with low pass at 80-100hz. Out of the 3 boxes, the 2 cubic feet tuned low had the best output and overall sound in the car (sounds better after cabin gain kicks in) and I normally dont like low tuning myself.
Basically one of the worst sounding subwoofers i'd ever had the displeasure of owning. They dont have a normal sound to them because they arent built like normal subwoofers. Every other sub I put in sounded fine in those boxes and played a much wider range and had no problem reaching up top or down low.
"
Air Suspension System
For its revolutionary 3.25 inch deep Shallow Subwoofer, Pioneer created an Air Suspension system effective within a 70% smaller-than-previous enclosure that locks air for a spring-like effect between main and drive cones moving in unison. The result is consistently clear,powerful,wide ranging bass response. The subwoofer's dual cone structure maintains better sound linearity and reliability after hours of hard use than conventional subwoofers with "spider" structures to keep the voice coil aligned. This incredible technology is applied to this new generation of high output, high performance subwoofers."
Basically weak motor, lots of MMS due to a double thick and heavy carbon cone design, you'll chip your screw driver before making a crack in that cone. Most people that own this sub know that they perform much better in a bigger box and barely get any performance in a smaller enclosure, some guys i know actually get great results with 2.5 cubic feet at 32hz as well.
OP, you can toss some stuff in to fill the box, anything solid will do, it will take up airspace and increase your tuning.