If your T400 is DC coupled, you might want to consider how likely the tube pre is going to pass on DC.
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.