I have had this same thing happen (many) times, and apparently the culprit is the DAC, specifically the Crystal Semiconductor 24/96 upsampling chip, which can sometimes become "confused". When it acts up, the high frequencies are affected as described. To reset it, try toggling to a different input on the DAC, and then back to the input you were using, and this will allow the DAC chip to reset. If this doesn't work, power the DAC off, then back on and it should clear. I have had this problem occur when changing from one CD to another, and apparently it is triggered by interruptions of the DAC's "lock" on the bitstream. Hope this helps, because it can be frustrating.
What the Hell Happened?
Last night I was listening to my system. As happens occasionally, the top end was shelved down from normal. Some high freq. air was missing, music was a little closed in sounding. Like I said, this has happened every few days for about 2-3 months. I've always thought it was either unidentifiable audio gremlins or my hearing was a little congested that night(I am 51 yrs old now). However, when I picked up the CD case for Someday My Prince Will Come, Miles Davis, my finger touched the metal top of the CD transport and there was a slight "snap" sound and immediately the music opened up! I could easily hear it while standing in front of my equipment stand! I sat down and listened to the last two CDs I'd played and all the high end extension and air was back, and it stayed the rest of the night!! Anyone have any idea what happened and how I can prevent it or correct it when happening again?