Starting with a general statement: Measurements are done as close as possible to what would be a scientific measurement process (Configuration and tests are documented, are reproducible by a third party, tests results are published, known assumptions and limitations are mentioned as far as possible).
Now the claims:
Statement #1 : Measurements are done in recording studio environment. Namely, there is no control on CEM/EMI interfering with measurements apart from standard studio installation best practices in terms of proper ED grounding and shielding.
Statement #2 : One input and one output are used on each unit, there is only one pair of interface sample tested. Having a pair allows to check for a similar performance on both interface, to rule out obvious erroneous failures on one interface. It is understood that statistically processing those measurement among several interfaces and unit S/Ns would be much better, but is out of scope of this measurement procedure.
Statement #3 : XR18 has two high impedance instrument inputs which MR18 does not have. For the sake of comparing apples with apples, Input 9/10 and Output 1/2 are used as references on both units.
Measurement Gear
UUTs
MR18 S211000533C8H
XR18 XXXXXXXXXXXX
Tools