There's a lot to read there, Marty and I did spend some time, but this is what I think:
There is way too much 'patching', too many complications. Aim for simplicity and 'directness'.
Here are some radical things to try:
Instead of getting manufacturers of speakers to get you a way to tweak components like resistors, radically change the approach, and first work on your room.
Use REW and a good condenser microphone and make a proper diagnosis of the room. Then correct the room response appropriately using organic, acoustic means. Use DIY acoustic panels as well as DIY bass traps.
Get rid of the piano in the listening room: it has strings and has a very good resonant body, the glass behind your listening position is not very good either (this judging from a linked-to post from this Original Post, so I have read partially two or three threads already with the walls of texts - not that this post is any shorter, haha!
).
Any resonant body will seriously affect what you're listening to, including your floorboard for instance.
Once this is done and the room response is appropriately smoothed, you can consider clean power, all the more important if you're bent on using DSP correction.
DSP correction for me isn't a panacea and should be avoided at all costs, unless there are pesky peaks and troughs that haven't been corrected by the organic, acoustic room treatment. It's a last resort.
It's a last resort for a couple of reasons: each DSP component will have its own signature. Grounding issues with the added component can complicate matters
Now, for clean power and other components, you will also want to consider vibration isolation especially of the seismic kind. And when it comes to the ultimate, you will want to combine that with any AC filter box.
I read that you are already very aware of tubes, and like the mid-range, so here a SET tube is all that is needed, nothing complex, just the sweet spot.
Additionally, and more importantly for me: I am not in agreement about the importance of frequency response in a system. For me, this is a solved problem and has been for many years. It is far, far more important to get a proper time accurate response, especially for timbral, rhythmic and spatial accuracy.
In short: Attack Transient Response > Frequency Response.
Now, having said that, continuing on the approach of finding boxed speakers + crossovers and pursuing manufacturers to provide flexibility for tweaking may not be the best - they won't agree: they spend time and money to tweak the components for their brand sound. It won't happen.
This requires a different approach.
The different approach is to use a crossover before amplification and then amplify with the best for the job (frequency-wise for you), and that also includes having amplifier outputs connecting directly to the drivers.
Instead of boxed drivers, try open-baffles, where usually you are able to tweak or integrate your own crossovers. Have someone build one for you if you don't want to build yourself anymore.
Now, seeing your mention of DSP processors it makes me think maybe you haven't heard high-rate DSD?
High-rate DSD (like DSD256), combined with a native DSD DAC and SET tube amp (or a combo of SET Tube + SS for low ends) is what gives me the purest sound.
Explore this route: use existing PCM if needed, up-convert in real-time to high-rate DSD into SET Tube amps (or a Tube + SS combo) and if processing is needed, use HQ Player as it can process native DSD natively or it can up-convert PCM to high-rate DSD (you'd need a native DSD DAC). Best thing is just to play native high-rate DSD in a native DSD DAC with the room acoustically treated.
Crossovers in software form on a computer can do wonders too if you bi-amp (you can tweak the time delays if necessary there too).
Nowadays, there is no need to spend an inordinate amount to get fantastic sound. Yes, you can always spend a lot to get whatever you want, but if what you want to approach is live players in a hall, you don't need DSP to correct the response: that's like applying an ointment on a pimple: it can hide it for a while but the pimple is still there as is the underlying cause.
In short:
0. Attack transient response first rather than frequency response (the latter is a side-effect of your room and the piano in it as well as potentially other things like the windows behind the listening position)
1. Clean Power (and grounding)
2. Isolate Vibrations
3. Diagnose and treat the room first organically and acoustically - lose the piano in the listening room. Only resort to DSP in case there are big peaks or troughs remaining.
4. Use Tubes, crossovers before the amps, connecting to the Open Baffle drivers directly. Have you tried Marchand cross-overs?
5. Try high-rate DSD + SET Tubes (or PCM, but with HQ Player ideally) + bi-amplication for the lower freq 'slam' if necessary
Think for instance of having a great acoustic musician at your place (Yo Yo Ma or Al Di Meola for instance). Would you tell him: "Hey, instead of listening directly to you, I'd like to 'mic' you and then DSP process it to 196kHz and listen to that instead.
Would that fly?