Haha, I don't think my 8000 system starts to get loud until the volume is about 4.0. Even at that level, you can still tell that the loudness function has an effect - when you switch it in and out, you can hear a small difference. I think the BM8000 tapers off the effect as the volumes increase. The loudness function does boost the bass quite a bit when the volume is very low (0-2.0ish). Too much I think. But I like bass, so I can live with it just fine.
The BM8000 does have a large amount of reserve power, so the loudness function doesn't seem to rob too much from it at high volumes (5.0ish).
In my opinion, with the MS150's backed up in the corner, I'd prefer the loudness function off, and the bass control at +1.5. The MS150's are about -6dB anechoically at 40Hz, and this is the frequency of bass boost or cut on the BM8000. In the corner standing position, you'll get the maximum about of bass reinforcement, but also with the maximum amount of undulation in the frequency response.
I find, to my ear, the MS150's sound the best out from the corners a bit, with a smaller amount of proximity effect reinforcing the bass, and then with the BM8000's loudness switched in. I then leave the Bass and Treble both flat. Then I sit as far away as I can, about 17 feet or so in my room, with my chair up against the middle of the opposite wall. Sitting here reinforces some of the upper bass punchiness that you loose listening out in the middle of the room, while the lower bass is what it is, since the wavelengths are so long. Most of my listening takes place at volume level 3.0 - 4.0.
Of course, it all depends on your room too, haha! How big is it, what shape is it, where do you sit?
Like I said, the BM8000's loudness function boosts the bass too much at low volume levels.
Sorry if this is more rambling than help.
Jon