NT-505 XLR → Jeff Rowland balanced preamp → Threshold amplifier (RCA input) - there is sound - This connection worries me a lot.
It depends on the Threshold amplifier input circuit design, some amplifier just simply use a spst switch to connect/disconnect RCA input to XLR pin 2.
If that is the case, you need to pull out the XLR connector from the Threshold amplifier input, only connect the tube preamp to the RCA input of Threshold amplifier and switch the input selector to unbalanced and see if there is sound.