Looking for a low profile sub to fit in a shallow box in the smart, any ideas?
No specific recommendation, but here is a nice general one:
Go with a 10" subwoofer rather than a 12" or larger.
With a 10" sub, you can set the crossover at a reasonably high frequency (I cross mine over at 250 Hz) and get a rich, full tonality rather than the "blumph blumph blumph" of the larger subs. And, for your installation, the 10" subwoofer will be shallower than a 12", and of course will be less expensive too.
If you have the room, two 10" subwoofers in a common enclosure will give you acoustic coupling and allow frequencies as low as a 15" subwoofer, while still retaining the responsiveness and tonality of the 10's. The downside is that you won't get as many decibels as you would with a separate enclosure for each one.
"There Ain't No Such Thing As A Free Lunch"