The sound is very neutral with very smooth highs. The brightness filter works by adding back the highs taken away when crossfeed is engaged.
Headroom amps are famed or destested for their ability to create a surreal, 120 degree soundstage which is scarily realistic in prescence; others simply hear crossfeed as muddying the sound and boosting the bass, an illusion created by the crossfeed mechanism headroom uses.
I, myself, love the amp's sound with crossfeed on: it's extremely natural, very easy to hear imaging, and above all is perfectly realistic if you want to hear your music in front of you instead of unnaturally spaced out with left/right blobs. Ear fatigue caused by the blob effect simply disappears. The effect is more noticeable and 3Dish with the higher end amps and better source material.
Give the headroom amps a try. I'd suggest the maxed out home, with a reference module added, and perhaps a stepped attenuator. That will sound great, and you might like it over the XP-7.
One drawback of headroom amps is they are headphone-dependent. They seem to sound better with certain types of headphones than others, although the effect isn't that extreme. The Emmeline series is supposed to be "sweeter" sounding. I don't know if that makes them less natural or not, but people seem to have overall positive statements using different types of phones from different companies.
Conclusively, Headroom amps are great at doing one thing over all others: passing on the source signal in as pure and unchanged a state as is possible. They are designed to work with a powerful crossfeed mechanism built into all of their amps, and are easily customizable and can be fitted to your very needs.
Consider listening to one of them; if you like what you hear, go for it. I tend to prefer the Headroom amp sound more than the other models I've heard (E.A.R, Grace, Sugden, Corda); that's my personal taste. I'm one of those Headroom/Sennheiser fans. I'm definitely biased towards their amps, but you should decide for yourself what they sound like; lots of forum members have never heard them.
Cheers,
Geek