Well,
After using both the Sonica Theatre, and now the USB Transit, I can say they both work great for 5.1 surround sound pass-through and other 5-channel sound output.
The Transit driver/preference panel is very sparse compared to the Sonica Theatre, though. But that doesn't really matter to me, as I do sound adjusting on the Dolby decoder and on the iTunes equalizer. And both units seem to provide an equal amount of automatic detection of true Dolby signals at a decoder via DVD movies. The Matrix Reloaded sounds way-cool with either.
I think I like the Optical/Toslink connector and sound transmission on the Transit even better than the Coaxial/RCA cable of the Sonica Theatre when it comes to 5-channel, non-Dolby music, which is what I listen to most. Though, at first, the Toslink cable to the Transit made some of the speakers sound like they had a faulty connection, but it settled down nicely after about 30 seconds. Maybe the fiber optic Toslink connections tend to sensitive upon initial hook-up, I don't know.
It was not possible to keep the Transit and the Sonica Theatre and their drivers properly connected to the Mini at the same time. I wanted to switch between the two connections to see which provides better sound: Optical vs Coaxial.
So, it's only my somewhat subjective opinion, based on memory of about one day of difference, but I think the Transit does sound a little better via its Toslink connection that the Sonica Theatre via its Coaxial connection.
Peace to you all,
C. Livingstone