I'd have to say the first thing I'd consider is that we're looking at a change of 4mV here, which is pretty small. My first priority would be make sure I knew what the measurement error of my multimeter was. Since the E30 seems stable, repeat the same measurement on it 10 times in the morning, then 10 times at midday, when it's warmer. Check that you get the same reading when you move the probe cables around a bit, etc.
But let's say all those come up the same and 4mV is outside the measurement error of the device. The primary environmental effect that will change the way a circuit works is temperature, as when a wire heats up its resistance rises. Modern high-quality resistors are explicitly designed to minimise this effect, and a good circuit design will balance out thermal changes as well. But it's worth getting out a hair dryer and using it to heat up the X16 then see if this causes any change in the numbers you're getting (I remember amirm did this on an Yggdrasil he'd been loaned and the guy who'd lent it had a conniption, but you can't do any damage unless the unit is exceptionally fragile).
I think the most important thing, though, is to consider whether the numbers you're getting really correlate with a perceivable difference. Get a track and apply a 0.1dB low-shelf filter to it in Audacity or another audio editor. I'm willing to bet that you won't be able to tell the difference between that and the original file.