I used to have a crossfeed preamp which I tried with several electrostatics and found I did not use itc.
The nature of crossfeed is generally to monauralize sound, i.e. blend two channels to reduce channel separation. This will tend to make intsruments recorded off to the left or right sound more centered rather than off to the side. I don't see the point other than with a very few recordings, generally early stereo, which have a wholely unnatural left-right spread, eg. some Beatles where the vocalist is off to one side. I personally don't find that type of stereo bothers me much, whereas I definitely don't like monaural sound through headphones.
Nevertheless, to each his own. I suspect that some listeners find it easier to mentally project the sound field forward, if it is not spread so far left and right. Also some crossfeed systems alter the frequency response, so the right system may help correct frequency response problems with phones or material. The whole process seem pretty hokey to me, rather like the various pseudo suurround set-ups sold with many boom-boxes.
On the other hand I do use a Sennheiser 360 virtualiser with my little Stax SR003 phones, for use with DVD's. I find it works very well with Dolby 5.1 encoded movies to give a better sense of depth. and that it generally reduces channel separation so I guess that it probably provides some degree of crossfeed as well as a proprietary, synthetic Dolby-type of sound simulation. However, I don't care for these tricks with regular stereo recordings. I am also a fan of Dobly Headphone, at least what I have heard in the Pearl Harbor movie. It is even better than the Sennheiser.
So I think you need more than just crossfeed to get any real depth added to stereo sound. If you hear enough depth with regular stereo than maybe you don't need anything else.
You may also be interested in the older Stax Sigmas, which are a pre-lambda design which mount the drivers well ahead of the ears and fire sound back over the ears. They give a better sense of depth than the various lambdas and have a somewhat attenuated treble which tends to avoid the "treble etch" which some people complain about with the lambdas although because they are less efficient and they tend to lack the punch of the lambdas.