I am currently using tube Audio Research Reference 2 MKI with SS Pass Aleph 2. They are working pretty well. Both gears are sweet, smooth and warm. The only weakness from Aleph 2 that I can think about is the bass control. However, I use a Genesis G-500 active woofer speakers to make up my weakness. I am also thinking about to upgrade my power amp and speakers to Pass XA160/XA200 and Cello Master Strad (passive) in the future. In my opinion, the tube preamp + SS power amp combo is the best way to get the warm/sweet tube sound and also maintain a good bass control.