The output connectors of the Massive Passive are balanced. I use TRS (from the Manley MP) into XLR on the Eventide. Either way it's a balanced cable. I really don't think there's a problem with the connectors, they are all very high quality… even if the signal is low in level, how does that explain the noise even when NO signal is present? Even more how does that explain more noise in higher sampling rates?
You say that the converters have something like 110 dB dynamic range. I have no trouble believing that but firing up a frequency and spectrum analyzer on the Eventide's output shows a level of approximately -70 dB, which means that there's something like 40 dB of noise!!! So you see I wonder why I have so much noise without even a signal being present. Just switching on the H8000FW makes a considerable difference. When I play something this noise is masked of course, but when the signal fades to silence, the noise is still there.
Nick, when you switch on your H8000FW and you have no signal that feeds into it, is it noisy? If you switch sampling rates, does the noise gets louder?