Hi there
in no way is the amp under powered for those speakers. The BM 1900 will happily drive most B&O speakers including the S75. I have a BM1900 and a pair of S45-2 speakers and the sound is superb with loudness on and bass and treble set at 0 . Don't worry about what is written on the labels.
I really suspect there is something wrong with either your speakers or your amp, just because you spent a lot of money doesn't mean there can't be an issue. The only way to really determine this is to test them in isolation of each other. You could start by testing the amp with headphones. Then the speakers on another amp and another set of speakers on the amp, it could also be the speaker cables.
most BM1900 will require recapping at their age, so if this hasn't been done and you can isolate the amp as a problem,this is a good place to start.
the s45-2 are pretty reliable and if the woofers are moving freely, unless they have been over driven are less likely to be the source of the issue.
drew