Well...it does not quite fit under the seat (height and width).
The seat would need to stay in back position without the option to slide forward.
And you probably would not be able access the screws anymore to take it out/put back in.

So the under-the-seat-sub-woofer option is eliminated, if even the world's smallest car sub won't fit.

I've now opted for the center console...needs quite a few modifications (taking the sub completely apart and "modding" the top of the console) and no clue about the final sound quality.
We'll see...it might take a while :D