Hello again,
Jim, there are several TIFF types/versions. This could lead to difficulties finding a suitable viewer. Someone here may possibly be able to help if you post additional information about the program in which the files once were created.
Generally speaking, the data fork of a Mac document is usually what is needed for a file to be read by a program on a PC. The resource fork, which gets lost during the transfer to/on a PC, is required for Mac applications et cetera. For example, if you try to pass an unprotected sea file through a PC, the file is not going to be self-expanding any more. For protection of Mac files in transit via a PC to another Mac (in order to preserve both resource and data forks), an encoding is used (MacBinary or BinHex). I guess that this should not be a problem for your TIFF files, though.
Could you transfer one TIFF file from the PB 180 to the PC on a 1.44 MB floppy to see whether you get the same result as with the null-modem transfer?
From the PC, could you send one (not encoded) TIFF file on to the iMac (via email or on a USB flash memory drive), and then try to use Mac viewers there? The shareware program GraphicConverter should be able to handle most image files. If necessary, as a pure experiment, the transfer via the PC to the iMac could perhaps later be repeated with a file that has been encoded for protection on the PB 180.
http://www.lemkesoft.de/en/products/graphic-converter/
http://www.macdisk.com/tiffen.php
http://en.wikipedia.org/wiki/Tagged_Image_File_Format
Jan