That was my guess.
So when using an unbalanced feed, one should always use the included adapters. They are not shorting two leads of a balanced connector together, they are in fact connecting to a different part of the input.
I'm not sure about that guys! If you look at the included adapters they are MONO sleeve TRS 1/4" NOT Stereo/balanced sleeve! The entire length of the ring shaft connects to the female RCA sleeve! Just by looking at the thing one can deduce that sleeve will short pin 1 and pin 3. It depends on how that hybrid XLR socket is wired though. I don't believe the pins are discrete for the TRS socket from the XLR pins on a hybrid. The jack prongs are just physically connected to the same pins for XLR balanced as TRS balanced. Please tell me the part number for the hybrid XLR and I can determine if it's a special hybrid XLR with discrete pins for the XLR and the TRS sockets. I don't believe such a beast exists though I could be wrong.
I'm very curious as to how the single ended or balanced ins are wired on the TC Preamp-3 board. I wish to understand how the TC Preamp-3 board determines if it's receiving -10dBV or +4dBu AND singled ended or differential signals as they are NOT the same thing! Signal symmetry as in single ended or differential is DIFFERENT and INDEPENDENT from "balanced" as it pertains to interconnect pinout/pre circuit! I have tried open and closed shield on pin 1 XLR A1400 on the RCA sleeve. I found that open shield as in pin 1 XLR in is tied to Mogami 2549 shielding but open at the RCA end with RCA hot tip to XLR pin 2 and RCA single ended sleeve return to XLR pin 3 produced WORSE hum! than XLR pin 1 and 3 shorting at sleeve! I'm am starting to think that pin 1 on the XLR in on the A1400 is tied to signal ground and NOT chassis ground when TC Preamp-3 board is in -10 single ended mode!
Please axioms don't fluff this off! I know you installed a new grounding bar in later revisions of this piece! Is not pin 1 on that hybrid XLR pined to this ground bar! I sure hope it's not pined to signal ground! It seems to be guys when the TC Preamp-3 board is in -10 singled ended mode! All I need for a hum free singled ended source to work from RCA to XLR is the TC Preamp-3 board to go into -10 singled ended mode and for the hybrid XLR pinout to be pin 1 chassis ground (NOT SIGNAL GROUND!) pin 2 singled end hot and pin 3 return. If I then used a two conductor with shield cable like my Mogami 2549 and pinout the XLR pin 1 shield pin 2 + pin 3 - and at the RCA end pin 2 to tip and pin 3 sleave with the ground shielding OPEN it should work best. I think the problem is that pin 1 is signal ground and NOT chassis ground when TC Preamp-3 board is in -10 singled ended mode. Considering the unit is a true differential designed as Jc sorta alluded to I'd assume that the XLR in terms of the TC Preamp-3 +4 differential mode is pinned out as XLR pin 1 chassis ground, pin 2 "+" pin 3 "-" as in a true differential circuit with same impedance in both directions. Again it's important to note "quazi balacing" with balums, VERY common because 90% of amps with balanced ins are NOT true differential designs, has led to the practice of pinning signal ground to XLR pin 1 because in a non differential design chassis is signal ground right!
Please read these article from raneSound System Interconnection Grounding and Shielding Audio Devices
PS If you really wanna take the red pill check out this article from Rane on the Linkwitz-Riley crossover Linkwitz-Riley Crossover Brilliance
It really nailed it in my head for me this week working on my own speakers!