AFAIK, there is a hpf in the output chain. Its cutoff depends on the setting of the 2nd switch (the 1st switch is for corner, wall, free placement and doesn't impact the PL output, only the BL2 sound). This 2nd switch does impact the PL output on the BL2. I and many others have heard this with our ears (or perhaps we've all been tricked by the placebo) - read the old forum posts around the time the BL2 came out.

For music, it's generally recommended to connect the BS2 to the source, and then the speakers to the BS2, and set this switch according to the speaker type to get the appropriate cutoff for the speaker. I believe the speaker assignments for each switch position are in the manual (but, of course, speakers created after the BL2 will not be there). I believe the switch position 1 is for the BS1, and 2 is for the BL8000s and a bunch of others (these are the two speaker types I've had connected to my BL2)... don't recall which speakers are recommended for 3. There are only 3 settings.

In typical B&O fashion, the technical details are not published (perhaps in a service manual?).

For Home Theater, you generally do not connect your speakers to the BS2 so the audio engine can control each type of speaker individually.