I'm going to go against the popular view here.... and for the record, I have a pair of Sopra 2 and a pair of Rels.
The B&W 801 D4 is a serious, high-end speaker that is tonally accurate, fast and tightly controlled. It will deliver clear, precise music. And before people start piling on, yes, it's not perfect and I'm sure you all have your prefs. Having said that, anyone who says this speaker is flabby or dull is mistaken, Looking at the specs, your model has 10" woofers, a lightweight mid and the diamond tweeter. The freq range is 15hz-28khz +/- 3db! They are not cheap. List new is over $50K. So congrats on that. Next, to the point.
First, the idea of putting a "pretty good sub" with a speaker at this level is ridiculous. It will not deliver the precision of the 801's. 1, 2 or 10 of subs is not going to matter. They will smear the currently pristine lower end. Take a great sound and F' it up.
Second, you have absolutely no need for a sub with these speakers. Get your system set up and live with it for a while. A couple of years at least. You'll learn where your weak links are and I assure you they won't be with the lower end. If you do eventually want to add something that goes to the basement, buy the very best ONE you can afford. Then keep an eye out for another to turn up on the market somewhere or save your monies for a second one. But seriously, you're not going to need a sub and adding anything but the best will degrade a system with these speakers.