Ampeg scr di screaming

I just got an scr di off Reverb this week. It was advertised as "Mint". It came in original packaging sealed in a plastic bag with manual. It sounds great thru headphones but when I try to run out to my amp I get nothing but high pitched wailing. I've double checked my connections. Running to my amp using the unaffected "thru" out works fine. Anyone seen this before? I sent an email to support this morning.