I own the Concerto and Stage DAC and can relate to everything you report about them. I also particularly appreciated the method you used in attempting to minimise the variables to a standard reference point.
I have just bought a Lyr and am about to pull the trigger on a Bifrost.
Could the faint hum in the BL combo be attributed to the tubes used? Does the buzz disappear or change in quality or quantity after you roll the tubes? Sorry if this issue has already been addressed and I have overlooked.