If you want to use a tube preamp you will have to run it single ended. The balanced input impedance on the Threshold is only 600 ohms. Any CJ, ARC, or Cary run through the RCA inputs will be fine. The 31SE, 32SE and 3iX can also be used, but not through the balanced inputs.
Conrad Johnson vs Audio Research vs BAT vs Cary
Just recently acquired a mint condition Threshold T400 and love its sound. It will soon be time to upgrade my preamp which is currently a Quicksilver Linestage 1. I'm considering within the $2500-$3500 the following - Conrad Johnson Premier LS17, Audio Research LS17, BAT VK-3iX, BAT VK31SE, and Cary SLP98. I know there is a bit of range here, but am interested to see who has done some direct comparisons, and who is pairing with similar solid state amps. Thanks.