Shouldn't all this be testable - with the mat vs without the mat? Why would you not want to prove the difference by being able to say which is which consistently? If there was such a difference we'd be all over it.
How can one tell if it is a different master apart from listening? Is there anything descriptive in a file download, or is there information on a CD cover/vinyl cover where one could see information about this? Or is this a daft question (very likely)?