I finally ran this test on my unit which is either a rev2 or rev3, probably rev2. I'd open it check which chips it's actually using right now, but I don't have the correct bit for those hexagon head screws without buying one first.
Seems I have the exact same problem as
@isabido, and I did notice earlier that the sound wasn't quite right when I tried to capture audio from an old MIDI synth module using the front inputs, but I thought the culprit might have just been my cables or RCA->TRS adapters.
Now what to do about it? Even if I could get Thomann or MOTU to acknowledge there's a problem and agree to a swap I don't really want to go to rev1. I wonder, does rev3 fix it, assuming my unit is a rev2? Is there a rev4 yet?
I don't think using XLR cables for line sources is a proper solution. The manual specifically tells to NOT do it, and the signal will be degraded slightly because it goes through additional unnecessary preamps. Accidentally activating phantom power while there's a line device connected via XLR to the front inputs is also a real possibility...
The unit is quite nice otherwise and thankfully I don't really ever need four line inputs active at the same time, but the gain adjustments available only on inputs 1-2 could be really useful sometimes because I have much "consumer" grade gear which output relatively quiet "line" signals, and sometimes I'd like to monitor the signal from them at a reasonable level without going through the host computer to apply gain in software.
Has anyone contacted MOTU support about this yet? I might try and see what they say. The unit is supposed to have 4 fully usable line inputs, it was always marketed as such, and we know the first hardware revision did not really have this problem.