Thats a very odd problem. I would first start with Onhwy61's recommendation. Assuming the problem persists, I can only guess, but a couple of things come to mind. If you made copies of these CD's, you may have some settings wrong. I know you can make CD's for karaoke. From what I understand, to remove the vocal tracks, by alternating phase between the 2 channels. If this is the case, even if there is no vocals, it can probably mess up anything in the same frequency range. From some of the instruments you list, I think its a possibility that this may be the case.
Also, while talking about phase, check your speaker cables very carefully. If you wire the speakers out of phase with each other, it will cause all kinds of problems. You won't hurt anything, but, for example, the vocals may seem like they are comming from one of the side walls instead of between the speakers.
The only other thing I can think of is a possible gain issue. Sometimes too much gain or not enough gain can cause all sorts of problems. Kind of like if you buy a high output MC phono cart and a standard MC phono stage when you should have got a MM phono preamp. Too much gain and things sound very bad and sometimes disappear. Given you have a lot of tube equipment, you can very well have a bad tube. It wouldn't hurt to check your bias either.
Beyond that, I can't think of anything else other than to make sure you read all the other posts because and I am likely wrong on this one.
