Looks like the woofer is a 4" Polyplas shielded driver, part #NS-W4SS, 4 ohms. If JBL can't supply a replacement try looking at Madisound or Partsexpress. Find a 4" 4 ohm woofer or mid-bass driver with a round frame. Check the actual measurements against your driver because not all "4 inch" drivers are the same size or bolt pattern. Make sure the depth will fit. You don't need "shielded" unless the speakers will be right up against a CRT (and nobody uses CRTs these days). Get a pair and replace both because the new driver certainly won't match the old.
Take a look at the Dayton Audio RS100-4 from Parts Express (or Amazon). The Hi-Vi M4N from Madisound "may" work but it is 8 ohms. http://www.jbl.com/resources/Brands/jbl/Products/ProductRelatedDocuments/en-US/TechnicalSheet/NSP1%20SAT%20ts.pdf
Answers & Comments
Looks like the woofer is a 4" Polyplas shielded driver, part #NS-W4SS, 4 ohms. If JBL can't supply a replacement try looking at Madisound or Partsexpress. Find a 4" 4 ohm woofer or mid-bass driver with a round frame. Check the actual measurements against your driver because not all "4 inch" drivers are the same size or bolt pattern. Make sure the depth will fit. You don't need "shielded" unless the speakers will be right up against a CRT (and nobody uses CRTs these days). Get a pair and replace both because the new driver certainly won't match the old.
Take a look at the Dayton Audio RS100-4 from Parts Express (or Amazon). The Hi-Vi M4N from Madisound "may" work but it is 8 ohms. http://www.jbl.com/resources/Brands/jbl/Products/ProductRelatedDocuments/en-US/TechnicalSheet/NSP1%20SAT%20ts.pdf