I print 8x10, 8x20, and 12x20 images with a 26-1k with no discernible fall off at all. 1.5 stops would lead me to think that the either the bulb was very old or the diffusion dots on the glass were gone, or both. I have heard of some fall off with people printing 20x24 with a 26-1k but no where near a 1.5 stop fall off. Mine seems to work fine within a 20" square. Although Dick Arentz probably has the latest and best exposure units available to him, if you look in his book there are pictures of him using a 26-1k.
Okay thats good news Robert, just hope when i get my Parker Countess Royal 1k unit (see atached a pic of it in the warehouse) that i dont have any problems as i really want to print larger. Thanks