Sorry for whatever I posted, someone got a hold of my laptop at school and thought it would be funny, I guess. To answer your question, I want to say I heard something about an 18" DCON which are rated at 300w. The obsidian 18's are also coming out
A lot of the 18's you are going to find are higher powered with strong suspensions (which wouldn't be ideal on only 500w). Look at partsexpress.com and see if they have any lower powered 18's