USAudio,
The answer to your questions depend on what kind of music you listen to and which crossfeed technology you are using. Meier crossfeed <> Headroom as they use different methods to achieve the same goal so they have slightly different effects. I presume the Grace crossfeed is different as well.
I for one use the Meier crossfeed and love it (both old and new types, I have a "Blue" modded Corda HA-1 and a Corda Opera) and find it indispensable for listening to a lot of the classic rock I own and for (most ppl don't know this) FPS gaming.
Advantages: Music recorded with extreme stereo separation (like the Beatles, Led Zeppelin, etc)where specific instruments or singers are exclusively in one channel (left or right) sounds unnatural and fatiguing without crossfeed. With crossfeed, since part of each channel is inserted into the other channel (after attenuating and delaying certain freq's of the signal), the soundstage and imaging becomes much more natural, coherent, and easy to follow from side to side.
For FPS gaming, 3D positioning and imaging is greatly improved. Directional and distance queues are more precise and more natural. To my ears it's a much more accurate solution than either CMSS3d on the X-Fi (creative or the prelude) or Dolby headphone (think Asus Xonar) - and yes I have used them all.
Disadvantages: For the Meier Corda Aria and later (I think that's when the new crossfeed got introduced) there is an ever-so-slight reduction in treble energy/clarity. It's VERY small though - for example, for me switching to a better power cord on the Opera made a bigger difference (which in itself was slight).
For the earlier Meier models , you'll get a slight but noticeable reduction in perceived bass presence, and a soundstage that has a little more depth, but is not as wide.