I am currently using a pair of Beolab 6000 with Cambridge Audio Minx X201 Subwoofer using a Y adaptor connected to an LG ultra short throw projectors headphone jack. The Projector is connected to an Apple TV 4K. In the past, I have made the same connection with sub and beolab 6000 speakers to an Apple AirPort Express.
1. I set the sub's cross over at around 80Hz. The lowest the BeoLab 6000 goes is 60. So the 60-80 is a small overlap that I feel adds the needed extra bass.
2. The Minx X201 is a super-compact sub with 200W power. I set the volume at a couple of clicks below max since the volume to both sub and speakers are regulated by the Apple TV volume/remote. If you have a more powerful sub you will likely need to lower the volume.
3. IMHO, the sound quality of this combination is better than a BL2 + BL6000 which I also tried. Plus, in this case the speakers turn on and off with the Apple TV, with BL 2 I had to manually turn on and off the trigger switch.
4. The Minx X201 subwoofer easily outperforms the BL 11 which I also had an was able to sell on eBay for well North of $1,000. That is crazy since the X201 can be had refurbished for $200! The main problem with the BL 11 is that you can't adjust the volume and it is not sufficient in my opinion.
Using a Y adapter has been fine for me...I also redid the woofer surrounds on the BeoLab 6000 and that also seems to have improved the bass somewhat. I am using a different material than the original rubber surrounds.