the OP is correct in suggesting that his specific question only applies to 2012+ models, because there is indeed a large plastic vent outlet under the seat which would prevent the placement of an amp there. OP, i encountered the same problem during my install and i have a solution for you.
all you need is four medium zip-ties. thread them through the screw holes in the amp's chassis, and secure the amp to the bottom of the seat. this method retains full functionality of the HVAC outlet under the seat with the added bonus of keeping the amp clear of any hot air which may be blowing out of your floor vent.
hey sonic, should make a post with pics of how i did this? i'm new here.