Energy in music drops with increasing frequency so although you have a 4 ohm dip in the mids, your amp won't have to deliver as much power as they would at, say, 30Hz and 4 ohms. Give it a shot - I think you will have no problems at all. I biamp my speakers with tubes on top and they sound better than ever before. However, B&Ws are not the best suited for that arrangement since their lower crossover is kind of high at 350Hz. But it is still worth a try.
Arthur
Arthur