My F1X was working fine a week ago, but after using it on a weekend gig, it's developed a buzz at the 1/4" full range output. Not 60-cycle hum, but more of a buzz. The same buzz is not present at the XLR-out either pre or post eq. I replaced the tube, but no change. It's loud enough that I can't use the full-range out on a a gig. Anybody seen this before, or have any ideas?