People who choose the Schoeps CMC 6-- xt amplifier over the regular CMC 6-- generally choose it without listening to it first, and generally because of two beliefs:
[1] that signal components beyond 20 kHz can enhance sound quality even if those higher-frequency signal components aren't directly audible; and/or
[2] that equipment with response beyond 20 kHz has capacity to spare, and therefore will have no difficulty handling signals at or below 20 kHz.
There's never been any repeatable scientific evidence of [1], despite many years of highly-motivated people looking for it to be true. [2] on the other hand can be true under some conditions, depending on what exactly is meant. But as sloppy generalizations go, it can be remarkably sloppy.
First, some basic realities: For a board populated mainly by males above the age of fourteen, many of whom listen to loud, amplified music with plenty of distortion (both deliberate and accidental) in it, I think it's safe to assume that just about no one here has undamaged hearing up at 20 kHz, let alone beyond. And very few playback systems can reproduce 20 kHz accurately, let alone higher.
Many speakers and some amplifiers, when driven at 20 kHz and above, put out distortion products that fall into the audible range. Thus whenever any effects from the presence of 20+ kHz signals are heard, it becomes a major priority to rule out speaker and in some cases amplifier misbehavior. But that concern is just what I find lacking in all studies to date (especially from Japan) that I have seen which attempt to show audible effects from preserving signals above 20 kHz.
--Anyway, an important thing to realize about the Schoeps CMC 6-- xt amplifiers is that unlike the regular CMC 6--, they aren't flat up to 20,000 Hz; they have a certain rise in the top octave. This means that any listening comparison between an "xt" and a non-"xt" setup can never prove that response beyond 20 kHz is audible, unless that rise is compensated for.
Also, the existence of this amplifier model (or its digital counterpart, the CMD 2 xt) doesn't reflect an endorsement by Schoeps of any theories about audibility beyond 20 kHz. It was merely a response to a customer's request which Schoeps was willing and able to fulfill. Being a small company, they are inclined to do that when they reasonably can.
--best regards