My personal experience with crossfeed has been that until I counteracted it with EQ boosts on the higher frequencies: normal crossfeed for me is akin to mono, it makes the mix less detailed and muffled/bloated. That is why I recommended the ifi implementation, it does up to 60% crossfeed while also delaying one channel by a very minute margin, but, most importantly, it adds back mid-high frequency energy that the original mixing seem to take out of our perception.
As per
Golden Sound measurements on the Zen Can:
@Resolve has also measured the effect that the 3d/xspace has on the Sennheiser HD 6xx/650 when fed through the OG Zen Can or the 6xx variant (the one I have), which makes the FR rotate anticlock-wise:
This 3D effect is what kept me from selling this amp, because although I have had others that measure better than it and that can provide more clean power at lower Zs, I could never replicate the compensation effect digitally. It certainly brings life to 60s "fake stereo" recordings or even classical ones, as they often benefit from more clarity.
Problems with the ifi implementation are that they vary per product, I think, so the older ones might not be the same.
Surely it is something I'd like to read more about, as it is ingrained in psychoacoustic and might be important moving on to the 3D processing DSPs like Atmos and so on (which also suffer from the congesting effect with HPs IMO).