Sounds like it may be a passive sub which if that is the case, should not be connected to the AVR, rather it needs its own dedicated amp. Otherwise, if powered, and the sub has both "input" and "output" spring connectors, you would connect the FL/FR speaker posts to the "input" clips on the sub and then connect the "output" clips to the FL/FR speakers.
Thanks the receiver has one port as you stated, but I think the sub has 2,
Would the Yamaha YSTSW216BL be a better choice for this receiver?
Or what else can I use under $150
Go to any electronic store, Radio Shack, Best Buy..
Get a RCA Y adapter, (2) Male RCA plugs > (1) Female RCA jack.
Also as advised previously you will need to get an RCA Male RCA > RCA Male cable long enough to reach from the 520 to the Polk.
Subwoofer install Instructions:
Plug the (2) Male RCA plugs (adapter) into the L/R line inputs of the Polk
Plug 1 end of the RCA Male > RCA Male cable into the RCA Female jack of the adapter
Plug the other end of the RCA Male > Male cable into the Audio-Subwoofer jack on the STR-DH520
Set the X-Low Pass Filter of the PSW10 to Max
Set the Volume of the PSW10 to mid-way
Set Phase of the PSW10 to 0
Set ON/OFF (power) of the PSW10 to AUTO
Power up the ST-DHR520
Power up the PSW10
Those are the basic steps but I would strongly suggest that you read/revue closely the Operation Manuals for the Sony and Polk to help U understand the setup.