What the Hell Happened?

Sounds like you've got a faulty element in your circuitry. Unfortunately, it's not only hard to identify, it's also intermittent. Could be anything from a malfunctioning integrated circuit semiconductor to a bad solder joint. It's not even clear that the fault is in your CD player. (When you get the shelved down highs playing CDs, does it also sound that way when you switch to other sources?) Reminds me of when one of my computer monitors would occasionally tint the picture yellow. I found hitting the side of the monitor with the palm of my hand always fixed the picture. No way to find the problem source, nor any need to, so long as my palm was available. Along this line, think about some way to spark your CD player whenever you lose your highs. Good luck.

What speakers are you using? I know that sound strange to question and all, but it could be your speakers...or not. Also, what time of the day/evening do you notice this phenominon? Reason I'm asking, is that I used to have a system that sounded WAY WAY different at night than it did during the day. It also sounded different durring week nights than weekend nights till a certain time! I firured it was that my power is WAY higher when everyone goes to bed at night in the neiborhood, and it's lower in the day when everyone's using their power! My voltage went from like 115 during the day to over 124 during the night! I also found out, by accident, not even mentioning the situation, by talkling to another speaker manufacturer, that my speakers (Electa Amators /SF) either sound great or bad, depending on how THE POWER hits them!!!! i'VE FOUND THIS TO BE TRUE WITH MY SPEAKERS SPECIFICALLY FOR SOME REASON.
Anyway, it might be something to consider. Also, I've noticed that some DIGITAL systems tend to get all congested sounding after time, and can use a good DEGAUSING!! This might sound weird as well, but I had a guy do a demagnatizing demo with a CD based Krell system one time, andthe difference was NOT suttle! I'm not sure how that works, but he degaused the system with something called a UFO audio tweek magnetic device, and the system JUST OPENED UP LIKE YOU WOULDN'T BELIEVE. I mean I was a believer (youcan buy em for like $200 or so.
Good luck

If you check the posts for the last few days you will find that your not the only one with this problem.

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.

From yr description it's probably static build-up that was released when you touched the metal surface. IF it was the DAC as Gasman suggests, another trick is to press "pause" and back to "play" again. This resets the clock -- and may serve.