[LAD] [ot] rme fireface: weird balanced output measurements

hi everyone! yesterday, i visited my friends' new studio, to help them shake out some bugs in the patch and fix a monitor problem. there i came across a really weird issue with the line outputs on two rme fireface 800s: we put a test tone out in logic (yeah, they run a mac shop), and i went to measure the outputs. in addition to really high fluctuations from one output to the next (with identical digital input signals), i measured huge differences in voltage on the hot and cold side, such as hot to ground: 2.00 V cold to ground: 1.82 V despite the fluctuations across several channels, this trend was pretty constant. so i figured, maybe this box has a problem with its negative voltage rail. they had another ff 800 in there, which we measured for comparison. same issue. i figured, maybe apple wrote an oscillator with a dc offset, so we applied a known-good test tone .wav file. same result. is my thinking flawed, or are rme really delivering such crappy output stages on such a pricey box? my multimeter is not a calibrated one, but it's in the 100+€ range, and i used AC true rms measurement mode. i understand my meter has a very high input impedance, and that's what line level connections should have, right? operating more or less open-loop, without significant currents flowing. or should i use a shunt resistor and measure across that? if so, what value is recommended? thanks for any insights, best, jörn