A ported box is custom designed for a port tune. In that PSW505 it's probably around 30hz. Anything you try to play below in that Polk box below port tune will be largely distortion and uncontrolled sound, because the box will no longer be generating resistance on the driver movement. The amp's dsp on the PSW505 is then designed to roll off hard frequencies below port tune to protect the subwoofer driver from 'free air' like play back below port tune which will bottom out the driver. The driver is specifically designed with technical parameters to meet the design of the box and amp it is mated to. So no you can't drop that driver in 20hz tuned ported box and expect any satisfaction, even if you swap the amp out - just as I can't drop my 67lb JTR Captivator driver in a sealed box and expect any satisfaction in a sealed setup if I craved frequencies below 20hz. My JTR driver was designed from the ground up for ported subwoofer box use with a tune of around 20hz. Polk designed that setup - driver, amp, and box to a port tune of 30hz.
You want lower frequency response capability - you gotta pay.
Klipsch RW-12D is tuned to about 24hz.
20hz tunes on budget sub drivers would require very big boxes in relation to your current box.