I haven't listened to any of the gear you list, but i do have recent experience with both the b&w 685 and the monitor audio bx5. I can say that the mid range sound is very different when comparing the two. I had the 685s in my system first, then went to the Bx5s, and the mids sounded...thinner to me. Some would simply call this more neutral, but after upgrading to a pair of quads, i think I enjoyed the B&w speakers more, in retrospect.
Whether or not the DRAN 5 can drive the bx2s, whether or not the all-in-one design of the b&w is adequate, you will be faced with the same difference in mid range sonic character between the two brands. I personally found the more warm presentation of b&w easier to listen to,more enjoyable. I also think that two monitor speakers are a better bet than an all-in-one design, it is more versatile, you can swap sepakers out, try different cabels, even swap out the dran 5 if you want.
Whether or not the DRAN 5 can drive the bx2s, whether or not the all-in-one design of the b&w is adequate, you will be faced with the same difference in mid range sonic character between the two brands. I personally found the more warm presentation of b&w easier to listen to,more enjoyable. I also think that two monitor speakers are a better bet than an all-in-one design, it is more versatile, you can swap sepakers out, try different cabels, even swap out the dran 5 if you want.