Trying to fix a crappy subwoofer

Most likely there is no electronic filtering and the 160Hz cut-off is due to the band-pass box.

I'm thinking the same thoughts. This "fix" is getting very complicated unless I find the same woofer.

Jimmy - 180 is way too high. Peddle it to someone who uses it in a cheap 5.1 and cuts it off at 80, but don´t throw good money to bait bad money and try to make it work wonders.

I'm fixing this for my father.
A local guy who builds subs for a part time living did some cool experimenting with cheap subs. He did three things that acctually helped them out considerably. 1. more stuffing, 2. very thick layers of putty all over the basket of the driver, 3. braced the crap out of the box (internally).

When he was done, he had a bunch of us over and A/B them with the exact same sub without the mods. We were all very impressed. The stock sub still sounded like a muddy one note wonder and the modded sub sounded like a fairly good music sub.

He only had 10 bucks in the mods. Polyfill from a faberic store and putty from the hardware.
"Only" 50Hz - so what? The lowest bass note, electric or double bass, is 42 Hz and very rarely is that cord played "empty". Unless you enjoy large church organs or electronic stuff, forget about the subcontra octave. 40Hz with real punch, that´s where alleged "20 to 20000" walmart crap can´t cope and a really good sub can.
As to that modded sub - making the cabinet as stiff as something very stiff on national stiffness day. YES!!! Imagine how much power is needed to make the panels bulge, and that´s the power missing where it ought to be there to kick you in the belly.