I'm not aware of any improvements to the bass response using the X-Feed but there is certainly no loss of bass with the X-Feed in the circuit. The bass is slightly more centrally located in the soundstage which may give the impression of "added bass" but this is purely psycological as the frequency response is unaltered. The X-Feed produces a natural directional bass as opposed to the normal Uni-Directional bass (unnatural) that most headphones output and this may also be a factor in the perception that there is "more" bass.
To sum the bass question up: The X-Feed converts uni-directional bass to directional bass so it is being perceived as a more natural sounding bass.
The soundstage, with the X-Feed in the circuit, is thrown slightly forward about 6 inches or so forward out of the head making it more like listening to a set of loudspeakers. There isn't a "widening" of the soundstage, if anything the X-Feed slightly narrows it, but there is certainly a shift forward out of the head with everything sounding more together. The left, right and centre soundstage is replaced with a more balanced natural soundstage.
I trust this answers your question
Pinkie.