3-Way Crossover's and Amp selection?

I'm not gonna make any recommendations, but I will say that tube amps will not necessarily 'cut off' your HF extension, or even your LF extension in many cases. Depends entirely on the amps in question, but there are lots of great tube amps which are known for their top octaves as well as their midrange. Remember, the division of the audioband into three 'ranges' is entirely arbitrary and without basis in physics, and any great amp must really be capable of covering the complete spectrum, or very close to it, with naturalness and cohesion from top to bottom. A design, whether tube or SS, which can only do great bass or great midrange, cannot really be considered a great amplifier, and I wouldn't use tri-amping with an outboard crossover as a bandaid to try and get around this. Your speakers and system will probably only sound their best if there is some kind of sonic consistency maintained across the spectrum, and running very different amps in the range above the low bass likely won't get you there.