Hi Remy

1st up there are a few sizes for underseat subs. it ranges from 6.75 inch, 8 inch and 10 inch.

for some cars, 8 or 10 inch sub will be too big till it will bulge out to the rear passenger leg room. so it will be good to check the size of the place you are planning to put it in and select the right size.

2ndly, how low you want your sub to boom... of course when it comes to low bass, the bigger the diameter of the sub, the better the low bass sound will be... there are different satisfaction level for each users when it comes to this... as some people can only hear to certain level of Hz and after that, none. some shops do let you try the sub but most shop does not allow it.

3. There is also another misconception between new/beginner audioholics, some users are looking for the "Bab. Bab." bass and some the "Dub. Dub." sound & some others look for the "boommmmmm. bOoooooOommm. (car shaker)" sound. here's the ticker! there is a big, I mean BIG difference between Upper Bass, Mid Bass and Low Bass... Find out which one you like, remember this is the KEY! to choose the right Bass for you!

Hope this helps.