It looks like you have tried only the Monsters in the XLR balanced configuration. Quality of connectors and the care taken in the soldering process are also important factors in the audio fidelity of the cable. Perhaps Monster doesn't use a quality connector or doesn't have their soldering process up to the same standard as on their RCA terminations. Or maybe you just got a bad set of cables (?)
The other explanations about a truly balanced circuit vs. a "quasi-balanced circuit", as others have posted, also applies.
The other explanations about a truly balanced circuit vs. a "quasi-balanced circuit", as others have posted, also applies.