I have a alpine pdx600.1 and was inquiring on a 15 inch sealed sub box for this I currently have a JL 10w3 but want more bass but not at the cost of clarity.  I have listened to 15's that sound great and I think going to a crafted sub I can achieve this.  I have looked at the X series but like the Q's and not sure if my amp can handle the Q series subs.  Any other recommendations are welcome. 

You can underpower a subwoofer. I still would run the X and possibly add a second later. Have you looked at the SSA GCON? That may suit your needs.

I have looked at the Gcon. Do you believe this to be a better or equal contender to the X? They look similar in design and spec.  The Alpine PDX has been a great amp has better power efficiency that most. 

I just noticed you are from STL. Nice, i'm an hour away from you. I like both subs honestly, and it would be a tough decision for me.

The Q has quiet a bit more xmax which maybe be beneficial. The recommended enclosure sizes are just a hair smaller for the Q. The Q is also built to order and you can customize it to your liking. 

After reviewing the power handling on the Q think I better stick with the X think I am going to go with a 15" Fi   X sealed 2.0 box with poly fill should sound good.

I'd rather get the biggest sub. 600 watts would be plenty for a Q.

400 watts more won't change anything for your ears. But it may sound louder than the X.

The Gcon is my favourite !

Hard decision