Sorry... It's been a
day. I think my brain (and your firewire) is going at USB speed...
It could possibly be the Powermac. From what I have researched, some users report slow (USB speeds) with some of the older Macs with firewire. You have had 2 different external drives and different cables, but with the same results. Unless there is a problem with the ports themselves or something else in the Powermac...?
Sorry I haven't been much help. But at least your post is back on page 1...
Take the good with the bad!
Good luck!