I just purchased a JL Audio stealthbox subwoofer and a Alpine pdx 1.600amp.
The stealthbox won't be in for a week. Has anybody out there tryed these JL Audio stealthbox.

 

We just bought a 2007 MCS and it had one in it with a JL audio e1400D amp. Good bass sound if you like that sort of thing. The guy we bought it from had it installed, we just took it out and need to sell it. With the trunk so small, I just needed the room. It got 5 star reviews on the internet. You should really like it if you want the window shaking bass.

 

I've got one currently in my MCS, and I love it. I honestly think the amp you have is too much for the tiny little stealthbox though, I'm using the alpine pdx 2.150, which puts out 300x1. Alpine recomends using their 250/1, and rates the sub atr 300w. As for the stealthbox itself? I think it's the best option short of a costly custom job. The fit is great, and the sound is wonderfully matched to the car.

 

I've had one for 3 years now. Easy to take out for a day at the track.

Mine is powered by a Boston GT-20 200W mounted under the passenger seat.
Fits nicely and works fine. Not overly boomy or loud for the rest of my upgrades (component speakers up front - MB Quart driven by Xtant 4.4i amp and rear 6x9 MB Quart 2 way speakers driven off Nakamichi CD-400 head unit).

 

Just to add:

I have a JL Audio 10W7 fitted into a box in the back of my MCS and it fits perfectly and sounds amazing. Powered by an alpine amp and a kicker amp to power the harmon kardon speakers it works very well. Especialy sense its all in a little mini it makes it even better. So to answer your question, the JL audio subs should be a great fit for your MINI. you wont be dissapointed.