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
Get a used REL Strata II. Yes, you need a sub for home theater. A self powered sub doesn't strain your amp.
check out the Phase technology power 8 sub...

sound like just what you want ...specks are on our showcase at audiogon

Quest For Sound
New a HSU VTF-2
You DO need a sub for the LFE channel, its the .1 in 5.1 and is typically available as a line level output only from your receiver. Your Nikko should be more than enough to run a passive sub and would probably beat any internal amp in a powered sub, unless the powered sub were somehow equalized.
Swampwalker is right ... and I'll add that if you have crushing bass in your mains, leave the sub out of the music equation unless you have some perverse need for all that bass. I have always maintained that a sub is the hardest addition you can make to a stereo system. The crossover points and placement can drive you to distraction.
Another nice sub(s) is the Monitor Audio ASW-110 and ASW-210. They list repectively for $700 and $1000. I see new ones on eBay alot that go for $400 and $650.