Definitely not the Pass Labs (which, otherwise, are wonderful sounding) if your speaker’s impedance dips that low.
I had X260.8 monos and now I use the Parasound JC1+ monos for the same reason you mention.
The Luxman M900U should work, but, IMHO, I’m not sure if it is a good match with B&W sound. The resultant sound might be a bit bright and thin despite your tube preamps - but I guess that depends on your room as well.