Cold lights always need to be oversize relative to film format. For instance, I use a 14X14 V54 head
for 8x10 film. A good diffuser will help, but is not a substitute for the ideal grid size. I've noticed that many cold lights marketed for a particular model of enlarger per mechanical fit are not in fact ideal with respect to light distribution or eveness. The second problem is whether your enlarger lens itself is giving even distribution (doubtful). You should be able to compensate for both problems simply with edge and corner burn-in; but if you can't, you might need to look at the diffusion system itself and how to improve that.
Thanks for your reply, my spare diffuser is in storage about 23 miles away. I have pre-Winter stuff to do in it anyway so I will go get it. As far as the lens goes, it is a new condition Rodagon 150mm 5.6, I have not really heard of any falloff problems with it and it seems to be well regarded. Since I am going to be shooting and printing a lot of 4x5, I have no issues with doing whatever it takes to get the illumination as even as possible before I resort to corrective burning and dodging.
The light is often too strong at smaller prints too, so maybe the double diffuser is the ticket....
"I always put my fb prints in a filled aquarium on the wall.
No problem with dry down times..." APUG'r Willie Jan