Pioneer TS-SW841D Sub-Woofer, 500W max, 120W nom. 4 Ohm. It is 8 inch, shallow mount sub. Very nice condition. This sub is no longer made by Pioneer. Here's a link to Crutchfield's review;
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Can meet in the Baltimore to Cecil county area. If you're outside of that, I can make arrangements. I will ship but shipping is extra (USPS). I will accept PayPal.