loudest subwoofer set up w/ 500 600 rms?

What is the loudest subwoofer set-up you can get for 500-600 RMS?

Mine.

But seriously, loudest setup takes a lot of experimenting.  And sounds like crap...but you aren't asking for anything but loud.  If this were my test, I would start with a bandpass and use WinISD to help me find highest SPL  *in cuz's vehicle*  before I started cutting wood.  I'd use a series of test tones with another setup in that vehicle to find the best resonant freq.  Use that freq as your guide as you work the box dimensions.  I'd probably model it using two subs, and work out placement so that they are both firing from the same plane and in the same direction toward the back of the vehicle.

By the way, if money isn't a concern, why the limit of 600 watts?  Is cuz wanting you to set him up for comp?

My system sounds good AND gets plenty loud. ONE 10" sub, BTW.

If you want it loud, and only loud, stevdart has the right idea... But, it will only hit the certain tuning freq loud -the rest WILL sound like crap.

If he wants it to get loud at low freq's (which is considered SQ material), then a simple ported box will do you good. If he wants it loud, then do what stevdart suggested.