Divad, You might consider posting a new thread - I think a lot of folks not already signed into this one will open it and see it, and some of the more knowlegable folks that have posted probably may not be following it any longerby now.
I think it is possible (actually quite probable) that a tube amp with a wattage limitation of 65 in Ultralinear can drive a 90db speaker depending on the actual impedence curve of both the amp and the speakers and how they interact. Your nominal impedence and minimum impedence are not ideal for just any old tube amp, unfortunately.
Usually ultralinear is going to be a bit more forward/focused and triode will be a little more recessed and relaxed. Usually, but this is very amp dependant, and either mode can better fit with certain speakers that the other. I have a 'triode amp' which sounds neither laidback nor unfocused, and responds much as another amp I have when its in ultraliner.
Your speakers are fairly well know (to others) but you prospective amp is not. I would suggest that you post here and on Audioasylum and try to find folks who have actually used a tube amp with your B&W so you can find out what they need to drive them.
If you can find out, it is helpful to know exactly where your speakers minimum impedence droop appears (at some frequencies it won't be near as important as others) and if you can find out the maximum output impedence of the amp and at what frequency it appears. With that info there are some very knowlegable folks about who can give you some advise.
I hope this helps a bit, I know it really doesn't do much to specifically answer your question. But, if you are a gambler, you can afford a buy and sell loss should you be unhappy, and you are not a bass nut (thats usually where you will run into problems) you can always get a tube amp and drive it off its 4ohm tap and hope for the best. :-)
