The Vibe 15" 500W RMS Active Sub is good, my mate has one in his civic, was better than my Vibe 12" 250/300W RMS Active Sub!
Edit: And also, for great bass at low volume the 15" beats any 10" or 12" Active Vibes!
Look on ebay, in the shops they are about £300-£400 new, hold ur horses on them and wait for one to pop up on ebay!