Regarding freq cutoff: at the SOURCE the filters in most dacs "slam the door" pretty effectively. There will be nothing to amplify beyond 22-24 kHz. If you are worried about the amplifier itself creating ultrasonics this doesn't seem to be of audible concern. Some tweeters (not so many) can reach 30 kHz or more. In that case THD and IM could occur in theory at that end of the scale, but is this an issue?