I've been using BM15as for a few years. They're very nice but I have heard quite a few times that the passives with a good amp are a little better.
I haven't heard the passives but I can't imagine it's a huge a difference. They're just great speakers period. No fatigue, extraordinarily transparent (and trustworthy) midrange. The highs are not hyped like waveguides or certainly ribbons but they translate very well.
I guess my ideal would be to have ribbon tweeters for tracking so you get that highly detailed view to make sure you aren't missing anything, then for mixing and mastering I would absolutely prefer the Dyns.
One thing about the Bm15
in general: It's truly a mid-field speaker. If you're planning on cranking them up (and they sound great when you do) you MUST have good acoustics in your control room. These things go down to 35hz and you can basically get by without a sub (although I'd still like one) but once you get that loud you are going to reveal every little problem with your room.
I generally keep mine pretty close and play them low to do my actual work and then just crank them up for fun at the end. I have taken them to an acoustically constructed control room and cranked them up and they're fantastic under those circumstances but it's a different thing. If you didn't spend at least $100k on construction and literally build your walls for optimal acoustics....I would use them as if they were nearfields and keep the volume on the low side. They're still a pleasure to work on that way.
It's the price of less trickery in terms of the tweeter and a much wider sweet spot than any ribbon or waveguide.
When I bought my Bm15s I got to do a wonderful A/B of several monitors at Dale Pro Audio (thanks to Tim Finnegan!). I had a Coles switcher so I could fast switch to any of a number of pairs. I had 1031s, BM6a
, HR824s, some JBL, and SA2.5, SA3 (ADAMs). The BM15s won hands down, mainly for the most transparent midrange with the most natural sounding top and bottom. I also noticed that the midrange was noticeably more detailed on the BM15s than on the BM6s, even at low volumes. Believe me, I wanted to save the money and get the 6s but I couldn't when I heard the midrange differences. The crossover is actually higher in 15s, further away from 1khz.