I ran into exactly the same problem with a brand new XLS 802 two days ago while installing a new sound system at a night club. After unpacking and connecting the mains the amp just simply wouldn't power up, behaving just the same way as you described. Weird part is, that I happened to try turning it on about an hour later and it worked just fine. Nevertheless, I'm changing the amp for a new one after this episode... But I'd really like to know what's causing the shutdown? Any ideas anyone?