Ha yeah the iFi went straight back, if you're in the UK we probably had the same one!
My balanced DAC (M-DAC) is also fine, although I can use the RCA outs with no issues either.
I feel pretty confident the USB to optical is going to help so have a Douk U2 on the way from China. I will however test the Mojo via USB with my Windows PC when I'm home out of curiosity to see if it also has the high pitched squeal listening through the active speakers.
Also try running the Mac off battery (I had a similar issue with a Lenovo Thinkpad, but it only made the noise when connected to it's charger or docking station).