Best Preamp to drive Sierra Audio Amps?

I'm looking for a good quality preamp, either tube or solid-state to drive my new Sierra Audio Whitney power amp. The issue is that the amp as a low input impedance of 2200 ohms when using the balanced (XLR) connection between the preamp and amp. I was told that the EAR 864 tube preamp can drive load down to 300 ohms. Does anyone have any experience using the 864 with a ss amp with low Z? From a ss standpoint I was looking at the Sim Audio P-5 which has an balanced input impedance of 110 ohms. Would this preamp work without any sonic degradation? Thanks, Ken
I haveseveral friends that own both the Whitney and Denali amps from Sierra they have all experienced problems matching that tough impedance. Peter is gesigning a new pre amp to work with those amps and I hear the prototype is quite good The only pre amp I've heard that really brings out the best in those fine amps was the Audio Research Reference One That combo was SO good It made a monoblock Denali pair sound world class
Thanks Acuujim, What is the o/p impedance of the Audio research ref One? Most high qulaity tube preamps have an impedance of 300-600 ohms. This I thought was too high for the amps input Z of 2200 ohms? Is this not true? If not, than this opens up a number of tube preamps that maybe candidates for use with the Sierra amp. Thanks