Ehh.. I don't like that box a whole lot. Go with a traditional slot port. And 2.48 cubes? Is that before or after port displacement? What is the port area? 5/8 mdf??? If you want to go with a prefab, you need to find someone who knows what they are doing.
Now to your speaker options... When the speakers are given with numbers, that would be their "recommended" box size. If they say 2.0 cubes, then through factory testing, they believe that the speaker typically performs optimally in that size enclosure. Some speakers sound better ported, some sound better sealed.
With the subwoofer options you have given, it's hard to decide between them, since none of them are that high of quality, you really won't notice much of a difference. Ryan may step in and throw you a few options. I've finally been able to hear a few setups with American Bass subs, and I was rather impressed. A few other routes you may look at that won't run you hardly more if any would be DC Audio level 2 or 3's
http://www.dcsoundlab.com/woofers.html or even the Fi ssd series.
https://ssl.perfora.net/www.ficaraud...iew.shopscript
Could you link your amplifier on here please? A lot of times "500 watts" doesn't mean 500 watts rms. Then again typically memphis had their amplifiers name to their power ratings...