The Xonar Essence looked like a good candidate for a home built server except for the comment in JA's review concerning the 4800 ohm input impedance which "might give some tube preamps trouble". Any cards out there that might be tube friendly? Comparable to the Asus?
In this day and age, they'd likely expect that 99 out of 100 of all pieces of gear connected to a sound card would be solid state and be able to handle such loading.
All you can do is go to the given manufacturer's sites and check.
For example, EMU makes some interesting outboard boxes and all their specifications are listed on the site, including input impedance.