If your only would like to obtain deeper frequencies and release your BL3, a subwoofer would be a option. But I would choose sub`s like PV1 from B&W or microvee from velodyne before BL11. Or go for a BL2 (but only if you are using the DSS of a BV, otherwise you are not able to adjust it proper).
I bought the BL8002 because they sounded better for me than the BL3 (warmer, not so technic) and upgraded a view weeks later with a BL2. I did not choose the BL11, because I was not convinced of its pressure. A view weeks later I traded the BL8002/BL2 combo for a new pair of BL9.
I can tell you, that the BL9 is so much better than the BL8002/BL2 combo. If the BL9 is 100%, I would rate BL8002/BL2 only approx. 65%. And because for me the BL8002/BL2 sounded better than the BL3/BL11, I would only rate it 50%.
So, to make a short story long: Adding a sub to your BL3 is much better than listening without any sub. But you will not even come close to the "sight distance" of the BL9, even by adding a BL11 to your BL3.
But always keep in mind, that you always have to hear every speaker at your home.
BeoVision 8-40, ATV2, BeoSound 4, Front BeoLab 9, Rear Beolab 4000 MKII, Beo 4