High frequency drop-out with Alpine deck

I looked at a customer's vehicle yesterday that had an odd problem.  We installed an Alpine 9847 head unit and Sony 3-way speakers several months ago; also some 12" subs and an amp.  What's happening is that when the volume gets to a certain level, the highest frequencies drop out of the front speakers.  It sounds like the tweeters are switching off. Turn the volume down, they come back; turn it up, they drop out again.  I don't think it's a speaker problem because it happens on both front speakers at the same time (I didn't have time to listen for the problem in the rear speakers). 

I was guessing there might be some type of clipping protection in the Alpine head unit that was cutting in, but I can't find any reference to it on Alpine's site.  Anyone run into this before?