+1
I was also considering getting a crossfeed plugin at one point, but I gave it up, because I prefer the comfort o running Ptq in standalone mode and I'm not sure such a plugin will achieve the expected effect anyway.
Basically, I'm hoping to achieve a sweetspot in front with headphone listening, similar to the one when listening on good powered monitors.

I think that you can use this plugin in standalone mode loading it with the VSTHost (http://hermannseib.com/english/vsthost.htm) and redirecting the pianoteq audio with some virtual audio cable like VB-Cable (https://www.vb-audio.com/Cable/index.htm)

1. Has anyone experimented with using a crossfeed plugin with speakers, instead of headphones?
2. Do any crossfeed plugins let you modulate the crossfeed with, say, an LFO?
3. Do any crossfeed plugins let you split each channel into several freq ranges and have them crossfeed differently? I can vaguely imagine a realistic effect being derived from splitting each channel into several freqs and having them crossfade at different rates, modulated by different LFO's.

All in the interest of getting a more submersive, sitting-at-the-piano feel. And for the sake of experimentation.

https://goodhertz.co/canopener-studio

The plugin uses sophisticated algorithms, perhaps different frequencies are mixed with different intensities.

This allows you to simulate the sound of the speakers through the headphones.
Maybe worth adding?