The crossfeed effect is subtle. The amount of the effect it has depends on the headphones used, the quality of the headphone amp used, and the nature of the recording.
A high quality headphone is going to let you hear the effect better than a lower quality headphone. A fully closed headphone is going to be different than a fully open headphone in terms of how the crossfeed effect sounds.
A recording that already has a very good and very natural stereo typically doesn't gain as much as a studio recording where the stereo is all created on a mixing board. So try a variety of different recordings. Older stereo recordings where hard panned to the left or right can benefit greatly (in relative terms) from some headphone crossfeed. For example, Grateful Dead "American Beauty" or "Workingman's Dead", some Queen songs, etc.
I don't know what sort of processing the J River crossfeed effect is doing. There are various models and methods for doing headphone crossfeed.
HeadRoom and
Meier-Audio have some info on headphone crossfeed implementation and theory. Both HeadRoom and Meier-Audio offer headphone amps with a crossfeed circuit. The HeadRoom and Meier crossfeeds are built with circuits so are rather simple in what they do. Computer DSP effects can do a lot more and can do more sophisticated modeling (
head-related transfer function) and processing.