Check your negs over with a quality loupe to determine that they are indeed sharp. Prime lenses and quality body or not, things do go wrong. A hasselblad and Zeiss prime will produce awful negs if things are out of alignment and it DOES happen. Drops, knocks, manufacturing/assembly errors...
After the camera variable is covered:
- Glass carrier with quality top and bottom glasses (as in those from the manufacturer). IMHO you are wasting your time with other combos as they are not reliable enough.
- Check enlarger alignment with a laser alignment tool. Sadly it does take something with this level of accuracy for the sharpest prints at large sizes.
I struggled with various issues getting v sharp 35mm prints, but now have a set up where the only limiting factor is lens performance. I too have a Durst Neonon 50 2.8 (Pentax made) and it is one of the sharpest lenses I have ever used in an enlarger.
Believe it or not, most enlarger columns are not perfectly straight. I check (and adjust alignment) for every print I make if the column moves a significant amount. With small prints, you can be in the rough ballpark for alignment and get a tack sharp print, but at bigger sizes, you have to be smack on assuming you don;t want to stop down to F11.
I make most prints at F4-5.6 on a F2.8 lens, but find with my Nikkor 63mm that F5.6 has a noticeable edge for prints larger than 12x16 in the outer field. So the message here is there are lots of quirks to get to know!