Yup, I experienced the same exact thing when I tried using a First Watt SIT-3 amplifier.
I ran XLR from Xilica XP4080 to RCA on the First Watt.
Monoprice 104784 3' Premier Series XLR Female to RCA Male 16AWG Cable
https://www.monoprice.com/product?p_id=4785
I confirmed that this is a 3 conductor cable. Signal + goes to center pin on the RCA and the - and shield are soldered together on the RCA outer conductor.
I experimented desoldering the shield and leaving it floating on the RCA end and it didn't make a difference. Whine was still present.
I only heard the whine from the Jubilee HF stock compression driver left and right. Not noticeable form the LF cabinets.
I disconnected everything in my setup and traced the noise to the Xilica.
Unplugged everything.
I didn't plug in any inputs into the Xilica.
I plugged in the First Watt SIT3 and connected the left speaker cable. Turned amp on. Silent.
I plugged in the Xilica and connected the Monoprice XLR to RCA cable. Silent
Turned on the Xilica and when the internal relay turns on after a few seconds I hear the high pitched whine.
Is it possible that this is an issue with the Xilica? Should I contact them? The date code on my Xilica is from December 2020. Curious if the original poster and I have issues with our units. Or is this just an inherent issue with using XLR to RCA with high sensitivity drivers?
I didn't hear the noise when I previously used the Crown D75a or a pair of Amp Camp monoblocks which both had balanced inputs. I'm Debating now if I want to experiment with the Jensen transformers or sell this amp and look into other amps that offer balanced inputs. Also considering other amps with balanced inputs; Cyrus Stereo 200, First Watt J2, Benchmark AHB2.