Do I need a subwoofer?

I would use at least 2 subwoofers for the best ultra low bass that even the B and W's are missing. SVS is a decent sub, but I would not get anything less than the 3000 series and I'm partial to sealed or servo controlled subs. You can get SVS with a return window in case you don't like them. What is your room size? In general the more subs you have the fewer the null or dead zones in the bass. I agree with the need for room treatment.

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.