Yesterday I installed a Kenwood KSC-SW11 under my drivers seat. It fit perfectly between the seat mounting points.
Kenwood KSC-SW11 Compact powered subwoofer at Crutchfield.com
'05 TJ SE, 2.4L, NSG-370, NP-231, D30/35, Procomp 2(+)", KM2 33x12.5", ZJ steering, JKS Quicker Disconnects, Airraid, Cobra 75WXST, super-comfy racing seats. On the garage floor: axles, JKS 1.25 BB, BMML, Rockkrawler T-case skid, .75" spring spacers...