My initial thought was to hook the amplified signal from the P2500S into the sub's high-level inputs and connect the speakers to the high-level outputs. I am fairly certain this will work, but is there another configuration that would improve my final output?
No, don't do that.
My first point of ignorance is that I do not know if passing my amplified signal through the subwoofer's high-level circuit will noticeably degrade my downstream monitors' signal.
Very much so. There is typically a 100 to 150uF electrolytic cap in there for the signal to past threw. Not something that you would want in the signal path.
Second, even if I wanted to, I'm not sure if I could get the appropriate low-level signal from my amp to feed the subwoofer. Is my equipment capable of that?
It looks like you can leave your settings in the "off" position to get a full range output from your amp.
Third, if I use the high-level input on the sub, will it bypass the need for the sub's internal amplifier or just reduce its load?
It does neither. The sub-woofer plate amp just senses the incoming signal it does not use it as part of the output.
Fourth, if the sub does use the Yamaha as its amp, I think I will still have plenty of power (250W + 250W) for it and two monitors---I doubt I'll put more than 50W each from them and the sub will need to blend with that level. However, I might have overlooked some reason that my amp is much to powerful or weak for this setup.
Your amp will not be powering or driving the subs at all. Your plate amp will be powering the sub, and you will be able to turn it up or down using the volume control as a gain setting.
Fifth, assuming my sub could draw its power from either the main amp or its built-in one, which would be a better option? This seems like a complex question, since I have no idea how to determine which high-pass/low-pass filters (in the sub or the amp, if they both have them) would damage my signal least. Also, I don't know if the Yamaha-amplified signal or the subwoofer's internally-amplified signal would sound better on the sub.
Again, it doesn't work that way. Your sub will only be powered by the plate amp. It will receive a full range signal from you power amp and basically covert it down to a low level signal. All of the controls on the plate amp will be used for adjustments.
Sixth, can the sub provide crossover circuit so the frequency overlap between it and the main monitors is attenuated to a smooth transition? Could my amplifier?
Run a pair of speaker cables from your amp to the N2X. Then run a second pair from the amp to the high level input on the plate amp. The N2X will roll off naturally at 12db per octave. The crossover built into the plate amp is likely also a 12db per octave roll. So you can then set the sub to play 40 to 50Hz and down and get a smooth transition between the two.
