Hello:
First, I would like to disclose that I am an authorized Burmester dealer and have been for some time.
Your question is a good one. While the price points of the 961 MK3 and the B30 are relatively close, the speakers actually have quite different "characters".
The 961 Mk3s are a bit more of a finesse speaker with the ability to exploit subtle textures and microdynamics extremely well. The B-series, by contrast are more of a "macrodynamic power house" speaker, well equipped for rock, and loud play. This is not to say that the B30 is not good with low level detail, it's just different.
Both speakers will perform at high levels with your 911 and will not concede any ability to play with extremely low distortion levels at any volume. Frequency response will be comparable (perhaps a slight edge to the 961 in Bass). Both still also carry a distinct Burmester clarity of sound.
Basically, the 961 is a tad warm of center compared to the B-series while the B-series is a tad cool/forward of center.
We have compared these speakers extensively in our Classic and Top Line systems, should you like further insight. I hope this has helped.
Auravis Systems LLC
Draper (SLC), Utah
(801) 558-1625
Regards,