I have exactly the same issue. Can you please advice me. I have the latest xps 13 9350 with qhd screen. Laptop's screen works just fine, but the external screen looks very ugly. Basically the same problem as described by the topic starter.
Please reply with the brand and model of LCDs that you have tested with the notebook and please include what resolutions that you are trying with the notebook and I may be able to help.
Specialize in Laptops, Mobile Devices