I remember everyone discussing this last year and at the time almost everyone said that they turned the 'Loudness' on...and infact B & O recommended that you left it on all the time and so we were wondering why then did they bother giving you the option of on or off?

I have a BV-7 32 which i use to play our music through too...connected to Lab 3s, Lab 4s and a Lab 2...and i will do some experiments with the 'Loudness' turned on and off! 

Soundproof, my room is medium - small sized with minimal materials to absorb the sound (leather couch, wooden floorboards, wooden blinds etc.) 

My BS 4 arrived with loudness set to on, and when i toggle it on/off at higher volumes, it doesn't seem to make an audible difference, so there ya go, it does vary with the volume (on the BS 4 anyway).