Hi Beomud
I used to have 4 lab 6000 in a suround setup and did after a while add the BL2 and the sounddifference was huge. As you very correctly write, the 6000 arent very good at handling the low frequencies and with the lab 2 attached it can concentrate on the higher ones.
However, there is a huge difference in the way the Bl2 adds to your audiosetup. In a surround setup it really improved the experience a lot IMO, but when listening in speaker 2 mode, the sub sometimes has som trouble keeping up with faster beats.
So because of your audiosetup, the succes really depends on your musicprefference. It will add value, but if it is enough value for the money i just dont think so. My advice would be for you to go to your dealer and ask for a demo. In this, make sure the Lab2 are configured the right way. Furthermore ask for a demo of different music.
Good luck
BV7-40 MK IV, BL5´s, BL3´s, BL 3500, BL7-4, BS3000, BC 6-23, BV 1, BS3.