Some extra comments, which I wrote as a reply to a PM,
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I have tried the x-feed with pop/rock, classical and jazz.
One thing none of them lacked with the x-feed is bass. I couldn't notice any drop off. I find no thinning or lessening of the signal using the x-feed and don't normally have to alter the volume after adding the x-feed.
Just to point out there is no switch to by pass the x-feed.
One other point of note: these listening tests were done using a better amp, namely Aos’s Sostenuto and the HD580.
On classical music: On music with 1 to 10 instruments I think the x-feed sounds great and removes headphone fatigue without spoiling the music. With more instruments (orchestras) sometimes I feel it get a little crowded. I felt this most with Beethoven's 9th which has a lot going on. I think most amps and headphones would struggle with Beethoven's 9th even without the addition on a crossfeed, I know mind do. Also on choral music I found the voices didn't quite standout as they use too.
Pop/Rock: Most of the pieces I listen to sounded fine with the x-feed. I felt it fell down more with bands like The Doves which is rhythmically driven music which needs separation between the drum, bass and guitars to get the full effect, with the x-feed it just started to get a bit muddied which started to covered the vocals .
Jazz: Well I did enjoy listening to the x-feed with jazz, but the sound it different compared to when the x-feed is not used. Jazz I personally think needs separation of the instruments to hear it at its best. When using the x-feed you just missed some of the minute details. But again you are able to enjoy it for longer.
Personally I leave the x-feed in for most music and find I more inclined to remove it for particular albums (which is not often) not genres.
I feel you will either leave it in all the time or only plug it in on particular albums that have extreme stereo.
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Mark