Regardless of power, a smaller driver has to move more to get as low, thats why it needs more power. A harder working driver will distort more. Distortion is bad!
SQ wise, Monoliths are very highly regarded in the £500 catagory as one of the best. At the original RRP of almost 1k I would hope it would hold its own against a Monolith, but such a price drop suggests it wasnt worth that price given competition such as the monolith.
It may be an equal, but physics is on the side of the Monolith with a larger driver in a larger an ported cabinet. There might not be much in it in the end, but unless size was an issue I would go with the BK.
As to whether 7oaks will have that price for any length of time, well why not look in store and get a demo while there