Will a 75x2 tube amp drive BW 804D?

I have to disagree w Marakanetz. Check out member Dougdeacon's system. He is driving 803Ds w a 60 wpc tube amp (2 el-34s per channel). The 804Ds are rated at the same 90dB efficiency as your 804Ds, in a smallish living room, maybe 14 x 16 or so. I've heard his system several times and I can tell you that the amp is more than capable of driving those speakers very, very well. I have the same amp and it does quite well w 89 db Merlins in a 14 x 23' room. Whether your amp and upstream components will deliver the undistorted signal that the 8XXd's demand is another question. If you have any HF nasties coming thru, they will reveal them ruthlessly. But as far as power goes, that should not be a problem. IMO, YMMV, etc, etc.

I guess you'll miss quite a bit of definition and detail on this combo, cuz in order to hear something in 40hz ballpark you'll need at least 200wpc. Using tube preamp can partially leverage your aim for the tube sound.

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.

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.