THe correct way of using the crossfeed is to isolate it within buffer stages inside an amp. That way, it will not interact with other componants in the signal path, and always work the same.
If it is not isolated, the x-feed/delay levels can change depending on the output impedence of the componants before it in the signal path.
Where the x-feed should be placed depends on the design of the x-feed. C.moy's modified linkwitz filter can be placed either before of after the amp, while Jan's circuit will only behave properly if it is before the amp. Of course, putting it inside the amp is the best.
But just because the x-feed is inside the same box as an amp doesn't mean it is INSIDE the amp! For example, if x-feed is used on a stock CHA 47 amp, it is NOT inside the amp. It is exactly the same as putting an external x-feed in front of a seperate CHA 47 amp. (except it saves the degradation of the interconnect, which is nothing compared with distortion caused by impedence mismatch)