I upgraded one of my 2 monitors to a higher res model.
SU lines on that monitor were irritatingly blurry / pixelated despite changing everything I could find in any anti-aliasing settings in SU, windows and my NVidia control panel.
I managed to fix it by:
- Searching for, downloading and installing a newer driver for my graphics card - this one is specifically tweaked for studio/creative work vs. gaming
- In Windows display settings, even though the higher res screen was set as “screen 1”, there was an additional box at the bottom of the options called “make this my main display” which I hadn’t yet ticked.
One or both of these things worked.
I assume because the setting were mistakenly based on my lower res screen as the benchmark instead of my higher res.
Weird to write something that’s already solved, but I will probably forget how to do this and come looking for the answer here. “Hey, Future Self, You’re welcome!” 
p.s. Oh… And Future Self, after you do step number 2, you may lose your pop-up option boxes off the side of the screen and SU will just keep dinging and dinging until you find it: ALT + Space ; M ; Press any arrow key (e.g. right); move mouse around without clicking until dialogue box flies into view.