I have TOPS on my little network (Mac Plus, Mac SE, LaserWriter Plus) but it
was not being used while I made the tests. (I do understand it's not entirely
passive, but in any event, I was using the scanner connected to the SE and
printing on the LW without moving any files via TOPS; neither Mac was published
for the other.)
It seems more likely to me that the scanners and/or software make different
sorts of TIFF files and that XPress doesn't recognize all of them (even though
PageMaker does, apparently).