GM...that looks AWESOME!

You probably know this already...but especially for an 18" woofer. BRACE that cabinet.
Easiest is probably just shelf braces running across at 2 layers with big holes in them...don't make it symmetrical and you'll help break up internal resonances a bit too...

What are you using for damping material?

And how are you connecting the layers? Glue and brad nails?
Solid choice of amp. You are unlikely to ever come near clipping that jobber. You just using binding posts to connect it to the woofer?

Geez, I feel puny with only two 15" subs.