I would go with one good sub instead of two cheaper subs. If you are going with the sub box on ebay for the A/C you can email the seller, and I believe he will make a box with only one chamber for a single 10" sub. Then you can use the other side to mount your amp.
To get the most out of your amp, you will want to run a 2 ohm load.
Here are some suggestions.
Alpine Type Rhttp://www.sonicelectronix.com....html
JL Audio 10W3v3-2http://www.sonicelectronix.com....html
Image Dynamics ID10D4 V.3http://
www.woofersetc.com/inde...=4993
The suggestions are based off the amp specs. I am not familiar with that amp. I suspect the amp may not actually give you 500w rms. The above subs could be underpowered by that amp. I would wait for some suggestions from Apple/Doc/Vballer(If he is still alive )