It is possible, I had a small 500W RF amp under the seat but I kept a 8" subwoofer in the trunk with a small box to preserve most of the trunk space
You would need a shallow subwoofer and the box would need to be extremely low profile.. more than likely not the correct cu. in for any subwoofer to adequately push out the full sound, so not sure having the sub underneath would be a good idea
I've always thought it'd be cool to install a low profile subwoofer into one of the big trim pieces on either side of the trunk (obviously putting proper insulation in there too) so you lose zero space
But what set me back was all of my interior is the ugly beige and a 8" black circle would look out of place I think