John: Well... yes.... bass is typically something that one prefers to "feel" more than "hear".... but depending on the situation (i.e. vehicle in question), more than one sub might be a decent idea to increase how much is felt. And i'm not talking about those stupid ricer's you see driving down the road.... where they throw an exhaust with a 5" tailpipe on a crappy old Civic they bought for $1,000 and think they have the hottest car in the world, so they throw some massive subs in it and crank it so loud you can basically hear all $1,000 of the car rattling.... including the exhaust. It may require more power due to doubling the output rating of the subs.... but it adds more punch to the bass. I really want to put a sub in my Brabus... but after putting a set of awesome freakin speakers in the doors, adding an amp to them, and swapping out the stock radio for an in-dash nav so i can have a bit more control over levels and such, i'm not even really sure i need one at this point. I could probably tweak it a bit more, since it doesn't seem that i always get the thump where it would be expected.... but it works freakin fantastic.
My '99 Dodge Durango on the other hand..... i had a pair of 12" subs in the far back (and no..... nothing major like Rockford Fosgate or that.... just Dual brand...), and that could THUMP! But the one thing i miss about that.... even though it could thump to it's heart's content.... the only thing ever heard rattling were the cabin pressure vents that Dodge so cleverly hid behind the taillights. Couldn't even really hear the bass from outside the car with the doors closed, and even with the doors open you practically had to have your head inside the car to hear the vast majority of the bass.
I honestly thought about trying to put those subs in the back of the Smart.... but i didn't keep any of the wiring besides what went from the sub to the amp.... so i'd have to go out and buy the power wires, ground wire, and RCA cable to hook it up... and that's a bit much just for an experiment.