Hey Leon .. I feel your pain... I have a simillar problem with my mamiya press, the contacts look great but the prints look really soft. I too checked the focussing plane and it looked fine. Here are a few ideas, If it's only when you enlarge your negs is it a problem with the neg carrier in the enlarger? if it's glassless maybe the neg is bowing somehow causing unshapness? I'm not familliar with the sl66 is it a 120 /220 thing, again just an idea. The only time I have had unsharpness(percieved) when processing is when I used HC110, all my pictures looked unsharp to me,though every other photographer who seems to use it, work looks fine?
Hope you find a solution to your problem