I have an 8 inch slim sub underneath the seat. Very simple and really adds to the sound. Im not a bass junkie i just like balanced sound. On a few songs (No hands comes to mind..) it can NOT hit the bass notes, but for everything I usually listen to its perfect. I have it grounded to the seat mounting point and its a powered sub. Super simple to hook up and takes up essentially no room. One problem is playing bassy songs for an extended period of time. I listened to it at a decent volume for about 2 hours and the amp over heated. Im sure if I would have had the climate control on Bi-Level it wouldn't have overheated though, because there is a vent under the seat.
If I were to do it again, I would get the 10 inch slim and put it next to the spare tire in the hatch area, under the carpet, or ditch the spare tire, and make a custom box for that area. I just hate subs / sub boxes that take up room in the trunk, especially when theres very little with a Supra to begin with.