Hello Andre,
I used to own an Audio Aero Prima Mk II SE DAC. I've never heard the Uther DAC, but I can give you my impressions of the AA DAC.
I don't think the high frequency of the Audio Aero is rolled off, but it is also not as resolving in the higher frequencies as modern good solid state DACs. There was a rather large mid-bass hump in the tube output stage of the Audio Aero that made the DAC sound quite warm and obscured the low bass to a degree.
The Audio Aero DAC was very well built, could double as a preamp because it had analog inputs and an excellent volume control, and was a pleasure to use. However, I thought the excessive mid-bass was an issue in my system, but might mate better in your system.
Alan