Do I need a subwoofer?

There is a lot of information in the bottom octaves that are related to soundstage and ambience, and other spatial cues.

As decently low as those B&W,s go on their own, adding good quality subs, will create a more open, larger soundstage, than without.

This tends to be more obvious with music that is recorded, with all musicians playing at the same time, and in the same acoustic space, such as: classical or jazz. But all types of decently recorded music will benefit

I just added a 2nd SVS SB2000 to my system and it's so much more balanced now. I'm running them as a stereo pair right next to my mains. I'm able to run them at a lower power compared to the one single so it doesn't excite the room modes nearly as much and still fills in a lot more evenly across the board with easily reaching into the upper teens . It takes a huge amount of the congestion out of the lower octaves that I never knew were there previously. As for your choice, I'd recommend sealed are definitely the way to go unless you have the space to do full open baffle servo subs.

Hello olfac87, Like you I found myself back in the research game about 12 months ago. I had to start from scratch and didn't even know what a dac was. Imop a sub can enhance any system and enjoy one myself. A friend once said to me "for food to taste good when cooking you have to f...with it." Again imop if you add a sub you should be committed to being an active participant with your system and willing to adjust levels depending on the production of the recording. If you do this it can be most pleasurable.

For my AV system I'm using two B&W 608 small subs along with  B&W 603 S2 towers, B&W Center channel and B&W 607 surrounds to make a 5.2 AV system.

This is completely separate from my 2 channel music system.

This little 5.2 manages to seamlessly meet AV surround stage for the most demanding sound tracks.  Point is 2 small subs are more than sufficient in a large room and easy to set up.

@olfac87 Firstly, I wish to express my condolences to you on your loss.

I also have just inherited a pair of B&W 801 Matrix S2s of roughly the same era from my uncle who is moving to senior living.

Agree 100% with elliottbnewcombjr and simonmoon

And what danager said is absolutely correct. In particular:

The good news is that doesn't require the most expensive subs and they really only fill in for room acoustics. 

I will add that they can also vary in size, manufacturer, etc.

I hear that active subs can also be thrown in to the mix.

So I put the 801s in my 15' x 19' (9' ceiling and wall to wall carpet) where I have a multi-sub swarm. I wanted to listen to these wonderous new speakers by themselves for a while.  Haven't owned speakers anywhere near this big in decades and these 801s just brought back many found memories.

After turning on the subs and some minor gain and frequency adjustments there was a very noticeable improvement as others have mentioned. 

The bass had more detail and finesse. But this improvement extends upward into the midrange as well. Bigger and more enveloping soundstage. Everything was just more convincing.

But for lack of ways to further (better?) describe the experience, I will just say that I really enjoyed listening more with the subs on.

Definitely worth your while to consider.

 Good luck and have fun with your search!