Sir Kenwood 3012 would be best compare to these 6 X 9 pseudo SUBs, you can get a nice small flat enclosure of 1 CU ft.
You can power it with your current Polk decently, bridge the rear channels.
Buy a 4ohm voice coil DVC SUB or a Single Voice Coil, if you do not want to buy a separate mono AMP.
A small 10/12" Alpine Type E or JL Audio 12W1V2-4 would be even greater if you can spend some thousand more and sacrifice some loudness for a much better output.