Vignetting from the P67 45mm lens with filters?

The simple test that Photopathe plans on doing will determine the problem. It does look pretty severe for light fall off, but it doesn't look even in all corners -- which seems odd, but that might be the print.

I have not seen that degree of vignetting when using the Pentax 45mm, with or without normal 82mm filters. There is a trend today toward filter "systems" with various types of quick change mounts. These systems extend the filter, lens shade, etc., and will commonly cause these problems for wide lenses.