Aggregate audio device issue

Hi,
Recently, I've been using guitar rig pro with the Aggregate audio device created in Audi MIDI setup. This Aggregate audio device was composed of the built-in input and the airphones app to output the audio to my ipod, which was connected to my audio system. However, I wasn't happy with the noticable latency, so, I went out and bought a usb sound card, and now, whenever I'm trying to delete the old Aggregate a. device to make a new one, It doesn't do a thing except deselect itself.
It seems as if I cannot modify the Aggregate audio device in any possible way.
HELP!

marc