Windows 10 x64
I too have had this problem - a clear difference between an image in C1 preview and the same image viewed as a TIFF in CS6.
I have just switched to a new 4k monitor (BenQ SW271 - love it!) and the problem is gone.
However, with some older software (e.g. Photoshop CS6) I had to make some changes since it wasn't scaling properly - in the case of CS6, text and tool buttons around the edges were tiny, even with UI set to Large.
This is the fix I found on the web and everything is perfectly scaled and tack sharp on the 4k now.
Since I no longer have the Dell HD monitor I can't tell if this fix would make have made any difference on it. But, it might be worth a try.
Basically, Windows default (I have no idea if there is a Mac equivalent fix, or indeed the need for one) is to allow a piece of software to set scaling. If there is a problem, as with CS6 on my BenQ 4k, you can override this to allow the system to set it.
This is how...
Browse to...
C: > Program Files > PhaseOne > Capture One 12....
Now scroll down to the C1 icon and right-click on it...
Choose > Properties > Compatibility > Change Settings for All Users > Change high DPI settings...
Tick the box "Override high DPI scaling behaviour. Scaling performed by...
...and from the dropdown menu below change it from Application to System.
Click Apply, OK and close.
Now restart C1.
Does it make any difference?
Probably not on a HD monitor.
But, I'm just curious.
If it doesn't, you can always revert to the Application option.
This is program specific, and will only change the option for C1.
D.