I own both pairs S120 and M150.

The M150 has more bottom but I have yet to really compare them as I have never done. 

Had MS150s and S120s - MS150s had far more bass which was detailed but liked the S120s more!

^^

Yes of course the MS 150 has more bass, it's a 4 way speaker :)

The S120 and the m150 are pretty much the same, the m150 is somehow larger.

That's why I am aking :)

A small comparison:

Volume S 120= 26,5 * 56 * 27,5 =40,81 L

Volume M150 = 32 * 67,5 * 32 = 69,120 L

So, the M150 must have a huger sound? But is the difference that big?

On paper maybe?

No joke aside M150's sound and react just as well S120 to the Beomaster in use. 

My current 2 Beosystems is M150 with Beosystem 6000 and S120 with Beosystem 6500. 

Still the behavior is quite the same and you have to play a little with bass/treble settings depending on the music or it will sound too flat. Sure the M150 does have more bottom but it is not that different at all.

I got the sounds fresh in my head and can which I have done give it some testing for the most valuable insight.

A B&O insider told me many years ago that the S120/MC120 was the best passive speaker B&O made. However best is a very judgemental term - I used to like my S75s which had a pretty uncontrolled bass but they were great for parties!

I have a working pair of Beovox MC120.1 speakers that I use with a Beomaster 6000 (Type 225x). At low listening (volume) levels the MC120's don't impress me much. But when I turn up the volume for serious music appreciation the MC120's start to perform. I have some Beovox MS150.2 and M100-2 speakers in the same room. I prefer the MS150's and M100's but I have no plans to get rid of the MC120's.  I like the more compact size of the MC120's. They fit in more rooms so they provide more placement options. But just remember to crank them up more to really enjoy them.

To me having both pairs and the options to turn them on any time I really don't think I would gain much at all swaping them from 6000 to 6500. I love the S120 sound and some volume is required from both.

I have used M150 with Beomaster 8000 and it does not seem to sound that loud compared to S120. Still its minimal difference really.