I don't really know if anyone has already suggested this since I have not gone through the entire thread, but I'd say that for the best piano reproduction, especially through the lower registers, you'd be much better off looking at something Open Baffle rather than a boxed design. Also, I think maybe you are looking for active crossovers rather than passive.
Passive crossover design and parts quality can and do start to play havoc with a lot of, if not all, instruments - but particularly with piano.This is because a passive crossover, even an extremely well designed one and that uses the very best parts, will act to clip off, or blunt, basically the top half of the dynamic peak. The natural clanginess of the instrument we all look for comes up missing or subdued. Oh the tone may seem very nice and all, but the realism still eludes. But, unexpectedly, when you go with active crossovers (I prefer digital) you smooth out the entire crossover region not only in terms of dynamics (no blunting at all), but also in tone, timbre, imaging and sound staging, as well. For anyone not having heard active crossovers, this total effect on the coherence of the sound would be revelatory...and for any instrument's sake, but certainly for piano as well - both throughout the low/mid crossover zone and the mid/high.
Generally, EQ can help, but balance alone will not net you realistic piano. IME, the underlying problem with tone does not seem to be the room or box resonances per se, but may be more related to woofer resonance in box design speakers.
In a boxed design, the woofer is under rather a lot of stress. The air pressure inside the box falls when the piston moves outward and rises when the cone moves inward. IOW, the air behind the woofer is exerting a (not always entirely equal) force in the *opposite* direction the signal is demanding the woofer to travel in. The woofer is stressed because it can't be in two places at once, so the woofer may tend to deform toward the edges under load. That sets up both distortion and resonances in the cone, especially as frequency falls and as excursion increases...again, not good for the deeper tones of a clangy (or vigorously played) piano.
OB, OTHOH, has no such disruptive force acting on the cone. The movement is equally unhindered in either direction and there are no real cabinet resonances to contend with, all of which lets the inherent performance of a driver that's well suited for OB purposes come to light more discernibly. Things like sustain and harmonics are noticeably longer and clearer. The only issue left with OB is the amount of bass cancellation at the lowest frequencies. Go with the largest woofer size here you can afford/accommodate, the biggest factor I've found with most OB speakers' low end response. I've been using 15-inchers for a while now and absolutely love them, with enough bass for piano or anything else.
To me, the ideal speaker system for piano would actually have at least three major ingredients. 1) OB bass with as large a woofer size as feasible. 2) digital active crossovers, and 3) some sort of good quality (that doesn't introduce other audible problems) power conditioning solution that will help tame any digital "nasties" your system may have and help things to sound nice and analog-like and it will help with things like the sound of the "air" in a piano's soundboard to come through and give it life. Not to mention the musical detail; the "clicking" of keys, feet working pedals or the incidental creak of a piano seat. It will also help flatten and extend the bass and give new clarity and air to the most emphatic "plinking" in the highs.
My only problem is that off the top of my head, I can't think of the best likely candidates in your price range, or otherwise really, since arriving at my own solution by way of DIY. But, maybe others here can point you to them.
Regards,
John
