Will a 75x2 tube amp drive BW 804D?

I would think it would; B&W speakers have a preference for being driven hard but 75 watts from a tube often equals considerably more from SS in actual practice. You can get the maximum output at one M by applying the usual formula of sensitivity in DBs x power output of the amp with twice the power required for each 3 DB increase in level. But unfortunately the only way to see if they will make you happy is to try them in your own room. Did you try the 805s in YOUR room. My own feeling is that they would be a better choice in a room that size. I have a pair of older 805s myself [I use to be a B&W dealer years ago] and think you would get much more of their potential in a room your size. Even if the 804s sounded better in a dealer show room they may not in your room.

I listen to N803 spreakers with a BAT vk55se. My room is about 13x20 ft. I keep the decibles between 70-75. No problem driving the speakers.