Bit late to the party but I second all that's been said. Fwiw, here's my long-winded 'amateur' perpective...
I bought the BM15A's back in the 90's. The local music store had a very friendly (or should that be 'cunning'
) manager who let me borrow loads of gear so I had both these and some Genelec 1031A's to test out side-by-side. They were both like a breath of fresh air, picking out details I didn't realise were missing on my old Alesis Spirit monitors, but it was a difficult decision to choose. It was only when I played one of our favourite commercial CDs through the BM15A's and my wife was reduced to tears, my mind was made up... although some may argue that was completely the wrong reason for choosing a mixing & mastering monitor!
The BM15As are classed as mid-to-near field, so as the guys said, you need a large room to do them justice. There's way too much bass for small rooms, but on the plus side you can actually hear very low rumbles such as air con, traffic noise etc. I plug the port hole with the supplied foam insert to reduce the bass response - the room resonates all too readily. This also gives a smoother tail-off in bass response as you remove the hump introduced by the port, so this apparently is recommended practice if your room isn't big enough to handle super low bass.
The BM15As are very heavy so are difficult to move around, especially if you use stands with spiked feet. These are the little things they don't tell you when they say "just move the speakers till you find the sweet spot"! As the guys said, the treble is very directional (as with any speaker) so you need to angle them so the tweeters point directly at your ears. I had mine on 42" stands to clear the back of the mixer, and if I didn't tilt them downwards, I found the treble increased whenever I stood up. The cabinet size dominates the tiny plates on my Quiklok stands so they always looked precarious when tilted so much, so I've since made larger platforms for them, and recently changed to 35" stands to reduce the amount of tilt required. The mixer meter bridge just covers the lower part of the woofer, but I think it's a reasonable compromise as bass isn't so directional.
Room construction is also a factor. I've used them in 3 setups in different houses. The first had stud walls, and sounded the best, presumably due to the the plaster board absorbing some of the bass. The 2nd house had concrete walls, and this was the worst, sounding very boomy.
My only problems with the BM15As was in hearing reverb tails. I found I was adding far too much reverb - this was apparent when playing the mixes elsewhere. I'm not sure how much of this was due to the room as I only noticed it when we moved to the 'concrete' house. Because of this, I always wondered if I should have gone for the Genelecs. At the time they were slightly cheaper, and their smaller size & weight meant they were a lot easier to lug around. I also read later that although Genelecs are very hard to get a mix sounding right, once you do it will sound right everywhere... but not sure how much of this was hype. Either way, it's too late now - they're very rare!
Interestingly, things have come full circle as my latest room has very poor acoustics - it's a loft space with a one-sided sloped ceiling, so I decided to bite the bullet and get some smaller speakers.. and ended up with some Genelec 8030Cs. These are perfect for the smaller space and I now feel I have the best of both worlds. I'm hearing reverb on almost any commercial recording I play! I've still kept the BM15s for their 'fuller' sound, although it's surprising how much bass the little 5" 8030s give out. The construction and tiltable isopod stands swung the argument against the newer M030s.
So apologies for the epistle, but hope that's helped. Good luck.
