I'm having a similar problem. Using catfish fonts to boot pbl 1.4.1 to evox on a 1.1 xbox (so I shouldn't have the focus chip, correct? I didn't open to double check). With the 'advanced av' connection box, my screen goes blank as soon as I start pbl from the dashboard. I have no problem when using the standard av cable or my friend's 'high def. av' box. Also, a third-party av cable (with audio l,r, composite video, and s-video) displayed the same problem. All were tested on the same television. It might just be a display issue, tho, because we could ftp into the xbox as if evox had launched.
Does pbl do any kind of av cable detection? Why would it bother?
p.s. I just recently switched to 1.4.1 - I had previously been using pbl 1.0 (!)