Plug the headset directly into the iMac.

Click the Apple Menu then press Option (alt). First menu choice should change from "About this Mac" to "System Information...". Pick System Information.

On the left, find USB. Then on the right find the headset. It will have information like this:

There may be an Extra Operating Current field as well, though this tends to be spotted only on devices pulling extra power to recharge a battery (such as an iPhone).

Note the numbers. Then unplug the headset, plug in the hub, and plug in the headset. Go to the File menu, pick "Refresh Information". Find the headset again now under the hub and note the numbers.

As others have said, it's likely a power issue.

http://support.apple.com/en-us/HT4049 is a good Apple support article detailing USB power and the information I posted above.