Younggun,
There is no such thing as fast or slow bass, all subwoofers will play the frequency sent to them by the amplifier. FYI an 15 inch subwoofer will play just as "fast" as a 8" or a 10". No matter what sub you are listening to, 100Hz is 100Hz, no matter which sub it is coming from. What you may be interpreting as slow bass is the lower frequencies the 15 would be able to play.
If you are going for SQ get a Lambda PB. If you have the space for it get a PB15, it is a very nice piece af equipment. It needs 3 a cubic foot sealed box though.
Trust me the PB15 can play any kind of music with so little distortion it sometimes actually seems quieter because of the low distortion level (no sub has 0 distortion btw) The PB15 is just as accurate as the PB10 and can reach lower frequencies and get higher SPL because of the much larger cone area. Like I said the only downside is that it needs 3 cubic feet. If you need the trunkspace, the PB10 like whiteSE's is also a great subwoofer, and also requires much less space.
http://www.stryke.com/SAL-PB15.html http://lambdacoustics.com/ The sub is listed at $199 US on thier site. You can call that best bang for the buck, but the truth is there is nothing comparable to this subwoofer for any price (within reason and yes I am aware some subs can reach higher SPL). The reason they are so "cheap" is that they are not a car audio company. Alot of what you pay for with CA is a logo, or basically what's written on the cone. The tech in this sub is amazing. You will not ever find a better subwoofer for your car at that price, or even for hundreds more. I am not here to push a brand here, if I were I would tell you to go get an Alpine sub (I'm an Alpine whore) This subwoofer is a product I truly believe to be the all around best subwoofer you can get and I think it would be a shame if you have the money and pass it up. Some may look at the plain black cone and think big deal but are these things made to look at, or to listen to?
This picture is just to give you an idea how much space the 3 cubic foot box takes up, I can still fit suitcases, coffee tables, bodies, and even lime and a shovel. Just so you know that's not how the car always looks, I had to throw it in there real quick for another topic for this same reason.
And the PB doesn't need alot of power to sound good, I ran it with my Phoenix Gold QX2180 (180 Watts MAX) and it hummed like a dream, usually I run it off an Alpine MRV-T757 (750W MAX and Alpine is the king) And I can make my back seat cushion move in and out with the bass. Most the time I tame it down though since I am an SQ guy.
If you've read this far without dozing off, good for you. It's like I said, I am only here to give people advice. It is up to you to take it or leave it. Hopefully Dave will be here today to either agree with me or try to be funny and say you should get a Pyramid sub...