Apogee ONE ground loop question

Sorry that this is not directly related to Logic. My question is about why one audio interface has a ground loop issue when another one does not in the exact same setup. Details:
Previously using an Edirol UA-5 as a USB interface. 1/4" outputs in the back connected directly to a pair of powered monitors. Monitors are plugged into a power strip that connects to a different outlet than my computer. No ground loop hum.
Recently tried to replace the UA-5 with an Apogee ONE USB interface. Changing nothing about my configuration other than 1.) plugging the ONE's USB cord into the port where the UA-5 was plugged in and 2.) using a splitter on the ONE's stereo 1/8" output to connect it directly to the powered monitors. Total hum and high-pitched whine through the monitors.
Leaving the ONE in place, I inserted one of those cheap 3 prong to 2 prong adaptors at the wall outlet for the monitors' power strip. Hum and whine disappeared.
So my question is: what does this tell me about these two interfaces? Which one has the problem? Given that the ONE is more expensive and of supposedly better quality, I would have expected it to be more tolerant of things like ground loops than the UA-5. Or is my understanding wrong? Is there some spec or sensitivity that is desirable but that renders the device susceptible to ground loops that I don't know about?
I don't know if I should be disappointed or not!

More info:
1. When I use a 2-3 prong adaptor on the the power strip that powers the monitors and connect the monitors to the ONE, a power outlet tester plugged into the strip shows a "correct" connection, which means it is grounded correctly.
2. When I use the 2-3 prong adaptor on the the power strip that powers the monitors and do NOT connect the monitors to the ONE, the power outlet tester plugged into the strip shows an ungrounded connection.
To my understanding, that's a lot of evidence that the monitors are being grounded through the ONE. Anything else I should try?
Strangely (to my understanding), when I plug the power strip that powers the monitors into the power strip that powers the computer, I get all the hum and whine again. In addition I get a mid-range-y constant chatter, almost like a fan noise. If I use the 2 to 3 prong adaptor to connect the monitor power strip into the computer power strip, the hum and whine go away, but the mid-range chatter stays. I had thought the hum and whine would not have happened since I think there was only one path to ground. Any thoughts about that?
Message was edited by: Will Rees

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*My Gear:*
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Also, If anyone has some good settings to use in Garageband for Pink Floyd settings please let me know.
Shine On!
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Message was edited by: mark.zelonski

Hey everyone. I wanted to make a correction on my original review about the Apogee "One."
I stated that sound could not be produced by internal speakers in my macbook. This is incorrect. Shockingly, I had the wrong setting in my system prefrences.
In addition, I decided to call Apogee directly about the "problem." The customer service was extremely good...very patient with me.
Good job Apogee!
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Apogee ONE ground loop question

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