BMHs are only really significant when the air flow is high because the gains at low air speeds are minimal. If the openings in the cabinet are big enough then BMHs are not needed.
I think a 6" duct to the cabinet is a bit of a waste of suck.
Relatively little fine dust ends up in the cabinet (especially if an under table extraction point is used) so high air flows are not need, just enough to clear the sawdust.
If I was to put a 6" extraction point on a BS I would put that under the table.
If BS extraction is being fed by a single 6" duct then at most there will be about 1200 CFM to play with. This is why 3 x 400 CFMs (3 x 100mm) ducting is often employed.
One of these under then table
One to the lower cabinet
The spare one is used by BP attached to the upper band guides.
This is because the natural wood cutting action warms the air and fined dust so it has a tendency to rise upwards, plus downward movement of the band drags air with it and the air bounces off the table top and reflects back upwards carrying fine dust with it.
Another useful location is here
https://www.woodworkforums.com/f200/i...13#post2070307