This depends on the cable, remember you are using 2 clock cables when you connect clock to both U18 and X26 Pro. With the right cables, you will be able to bring the mids forward back to where you like. I am using the same combo as yours and I can say that connecting clock to both U18 and X26 Pro brings the detail levels to a new height, up to high end audio standard.
For reference, I am using Cybershaft blue cable to U18 and Harmonic Tech to X26 Pro, and I also like mids sounding forward so that solo singer is like singing in the room.