The AD12-300 woofer Erik posted is an attractive choice indeed. As a drop in without any changes, It'll be fairly smooth and be 3db down @ 31hz.
The NHT1259 Woofer posted as a drop in (no box changes) will have a 1db rise below 80hz and be 3db down @ 19 hz.
The advantage of Eriks suggestion is that it is New. The 1259 woofer has been around for a long time, they are well sought after, tough to find and the chance of finding anything new will be even harder.