best sub for both HT and 2 channel audio for 500 bucks

I will be running four Martin (vintage) tower speakers, a B&W CC6S2 Center and am looking for a decent sub for 500 bucks. Is that possible? My HT receiver is a Yamaha RXV-3000. I need some sugguestions. This will be my first subwoofer ever. My Martins have Bone Crushing Bass already. Do I really need a Sub? I have a Nikko Alpha VI Power Amp (300 per side of good clean power), to possibly run a passive sub. What are the pros and cons of passive vs. active. Thanks for your input........Dave