Do I need a subwoofer?

Go directly to REL.  Do not pass GO.  Do not collect $200 (you'll need it).  Start with one and see where that takes you; don't assume you need more than one until you've spent the time to properly locate the sub in your room and dial in the gain and crossover settings.

These are a bunch of great posts. Glad to hear other 801 owners' experiences, and appreciate the REL suggestion. I will look into all this.

I very highly recommend a REL sub!  I currently have Zu Druids as my mains because I love them for music, but they don't create deep bass.  Plus I have a totally suboptimal, fairly large, open, irregular room.  I have experimented with a variety of subwoofers in an attempt to give some depth to music, and impact to movies without breaking the bank or taking up half my room.

A couple months ago I picked up a pristine used REL 212 SE hoping that the 2 -12 inch active, and two passive drivers could do the job in an acceptable footprint.  I had been reading great things about REL subs and figured this would be a great way to try them.

Make sure to read this all the way to the end....Because I initially connected the 212 the same way I had my other subs through the LFE on my preamp. and it sounded better, but not overwhelmingly so. I figured I had just reached that diminishing returns line for the room I have.  Though I was still hopeful things could get better once I connected the REL the way the manual recommends. 

The Speakon arrived a few days later so I had time to listen without it, and as previously mentioned, the 212 sounded good, but not enough different to justify the price jump over the SVS subs I had been using.  

Once I made the Speakon connection though I immediately heard a massive difference!  My entire system seemed transformed!  The bass was all I ever hoped it could be for music.  I played some favorite bass heavy music streaming through Tidal, and couldn't believe the difference.  Next I put on Deadpool Blueray to see what would happen with movies and I could feel my pantlegs flapping, and stuff on my shelves rattling the way I've heard other people describe!  And if that's all that happened I would have been very happy.

The totally surprising part was how much the soundstage widened and deepened.  Literally defies description.  It must be heard to be appreciated.  

But wait, there's more!  Seriously though, More!.  Having the REL connected through the Speakon connector and to my amp the way they recommend made the mids and highs sound better too.  Noticeably, obviously better.

I have been playing with a pretty wide variety of decent quality components over the past couple years and the REL 212 SE has overwhelmingly made the biggest upgrade in the sound quality of both music and movies for my system!

I cannot recommend any sub more highly than a REL!

REL is a good choice, SVS not so much for a music based system. The other good choice is Rythmik. Even their ported models are graceful enough for music systems but their sealed models are simply sublime. Especially check out the G22 (dual 12") and G25 (dual 15") models. You want subs that don't sound like subs but rather like extensions of your existing speakers and either of these brands are capable of doing that, and yes, you definitely want two. Budget for two from the get-go, don't "try" one and see where that gets you. And yes, three or even four are even better, but two should get you where you want to be. 

REL is a good choice, SVS not so much for a music based system.

Really @shooter41? What’s your experience with sealed SVS subs? My experience with my SB2000 in a fairly high-end, 2-channel setup is pretty damn impressive and I’d think adding another would be even more so, so I’d be interested in what specific experience you have to say this? Yeah, I’d probably prefer Rythmic or maybe even Rel, but they’d both cost much more for similar performance so for the $ the SVS subs get you pretty damn good performance. Maybe you could share what their shortcomings are, because failing a direct comparison to pricier subs I’m at a loss for criticizing them for bass speed, musicality, or depth. Please explain.