Is my amp clipping?

Damage is done, your amp may or may not have clipped at one time but I bet your teeter is in need of replacement.

Hmm...the tweeters still sound find at low volume. Has anyone heard of a CD player doing this?

No way the amp is clipping. Could be the tweeter or the amp/preamp. I assume its in one tweeter only? If so switch the speaker wires to the speakers and see if thre crackle follows, if so its the tweeter probably (or tweeter crossover).

Could be the volume pot, or any number of shorts, parts, you didn't mention if you had a preamp. Sometimes the volume pot has crud in it at higher volume settings of the rotation that causes just such a problem, just (unplug for a while) and blast it with some contact cleaner.

The noise is actually in every tweeter I tried -- even switched to a different speaker all together.

My preamp is by Van Alstine, I actually just got it as a used piece of equipment from his shop -- so it shouldn't have any crud...

So, noone's heard of Oppo CD players doing something like this?

Here's an easy troubleshooting step.

Plug your Oppo's L&R analog outputs directly into the Belles Amplifier. Turn the Oppo on, press the volume down button on the Oppo remote about 20 times to insure the built-in digital variable volume is set to a very low level. Turn on the Belles and play a tune. Once a CD is playing, gradually turn up the Opp's volume using the volume up button on the remote.

The Oppo has variable volume built-in, so you can bypass the pre-amp as a test. Actually, if you have a single source, you don't even need a pre-amp. Try it. If the problem goes away, its the pre-amp. If the problem still exist, try a different source or a different amplifier if you have one or can borrow one.

Enjoy,

TIC