I've got to chime in with my 2 cents. I have been listening to headamps for over 4 years now. I've auditioned or owned several higher end amps but nothing in the holy grail section. However, I've owned some decent sources and played them through some decent amps. I recently purchased a MOHR/SA from insomniac and began using the crossfeed feature. I've got to say that despite my "assumption" that I didn't need crossfeed due to widespread comment about it's usefulness/level of effect on sound, I have to say I absolutely love it.
For years I've read and assumed that "with better recordings" that crossfeed isn't all that necessary. Maybe that's the case for some but, for me, the crossfeed really does produce more of a stereo effect that is worthwhile and provides a touch of spaciousness that one hears with speakers. Perhaps it's because I haven't listened to headphones for a while and have been just using speakers over the past 8 months, but I really *like* what crossfeed does. It does not detract from the sound provided a little boost is given to brighten things up. It increases bottom end just a touch but isn't fat or in any way unpleasing to me. I like it so much that I will not listen without it as it really sounds more natural to me.
I can see how if you're not used to a good speaker setup (good amp & speakers) you would not be that moved by it. But after having listened to speakers and enjoying music how we hear sound naturally in our everyday world, I think it does what it sets out to do. And I keep hearing that the Corda stuff implements it better than the old Headroom flavor of crossfeed.
Either way, I like it and after years of being in the "I don't need crossfeed" camp, I find it adds to my headphone experience so far. It does decrease fatigue for me and I can listen through my 'phones while doing other things longer than I did before. If I don't want it, I can just turn it off. But, for now, it's a great feature to have and can easily be turned off should I change my mind.