I have a 2000 Avalon and I had the same problem with the sub...... I put in an 8 inch I had thinking if I used a 4 ohm it would work. Well speakers are different, you do need a free air and a lower watt may help. The radio is only putting out 32 watts to the sub. I look around and found one at partsexpress.com (
Parts Express:Goldwood GW-8024 8" Butyl Surround Woofer 4 Ohm) this is a link to the one I bought. If you look at the price you may think it's crap!!!!! But, you really don't need much. I do love my music and I can't tell a difference between the JBL and the Goldwood. except the price.