I went the cheap-route with the sub because of budget, and yeah only 1 under the drivers seat. But I see no reason why you can’t put one under the passenger seat. (Actually, it’s kinda on my extended to-do list to have one under each seat.)
Model of 8” low profile sub I bought: Audiopipe APLP-800 (aplp800) 400W Active Low Profile
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Fits great, sounds good. I’m 6’ 2” for the record, so it’s not like I’ve got the seat jacked way up. The sub height is 3” 3/16” and fits, which did surprise me, so rest assured anything less than that will fit too. Also I can’t speak to installation woes because I had an audio place do that part, but he told me there were no problems getting it to fit. He may have had to take the drivers seat off but don’t quote me on that (it was a while ago).
If price isn’t so much an issue for you, only space, I’d get the
JBL BassPro SL2 It’s lower profile and I’ve heard good things about the JBL low profile subs. I bet one of those under each seat would sound niice
I can get you some pictures later of how it looks under seat.