$600 for an amp & sub? Alpine has some good products, but $600 and it's only a 25watt amp for the other speakers.
Not really a new thing either, these kind of subs have been around for a while. Help Me DIY did something similar and put it behind the driver seat. Not really my cup of tea, I prefer to keep things hidden and cleaner.
If you want a 5 channel amp, look for the MB Quart NA2-500.5. More powerful then that Alpine and can be bought brand new for under $130. Get 5 1/2" subs/mids and replace the door mid speakers. I think there are better options then the Alpine unit, especially at that price.
I love Alpine products and was even sponsored by them many years ago for audio competitions. I still use their stuff.
Here is the MB amp getting installed. This is attached to a Kenwood DNX. Powers Kenwood 2 1/2" speakers in the rear storage tray, 5 1/4 components in the door and Focal's in the dash.