Problem using usb audio codec in aggregate device

I am using Cubase 7.5.10 on OSX 10.7.5. I recently added an Alesis MultiMix 4 which has a USB interface. The mixer works fine with the USB interface except when I try to use it in an aggregate device in Cubase. Using it directly in Cubase, it works fine. The problem is somewhat strange. It seems Cubase insists on setting the device, or interface, to 8-bit. If I change the setting to 16-bit as it is supposed to be, it gets changed back to 8-bit after a few seconds but only if Cubase is running. I can put the USB audio codec which is (system) provided for the mixer in the aggregate device in Audio Midi Setup while Cubase is not running, set the output device to the aggregate, and then play audio using ITunes and it works just fine. However, if I start up Cubase and put the USB codec in the aggregate device and set Cubase to use the aggregate device, one of two things happens. Either I get no sound in which case ITunes works fine, or I get a really lousy, hissy sound which seems to be caused by Cubase setting the USB Codec to 8-bit. When this happens, I get the same sound from ITunes. If I set Cubase to use the USB Codec directly, then Cubase works fine and so does ITunes which seems pretty strange to me. Setting Cubase to use the aggregate device causes ITunes to sound bad but setting Cubase to use something else makes ITunes work ok. This seems to tell me that the problem is in Cubase. Also, when I am getting no sound in Cubase, VST instruments are not getting any MIDI messages as determined by their MIDI indicators. External MIDI seems to work though. Does this sound at all familiar to anyone?

I found a post elsewhere from 2012 where somebody was having a very similar problem. The culprit turned out to be Rosetta Stone which was switching a USB Audio Codec from 16 to 8 bit. The fix there was simply to change the setting in Audio MIDI Setup. Unfortunately, that does not work for me because the setting immediately is changed back to 8-bit. This happens only if the codec is in the aggregate device and Cubase is configured to use the aggregate device. I am sure Cubase is switching the setting from 16-bit to 8-bit but I can’t imagine why.

Having this issue on 10.10 and cubase elements 8.0.35. Anyone knows how to solve it?