That way if I wanted to reprint something later with slightly different processing or do some editing to the DI'd performance and reprint, I can. My preferred work flow let's me track both the raw audio before the UA plugins as well as the plugin's sound into my DAW at the same time. I'm able to run a DI'd guitar in unison mode using the 610 plus the Marshall emulation, or the Sphere plugin and 610 mic pre just fine. The UA-610b mic pre plugin it comes with is great, as is the Marshall Plexi classic guitar amp emulation. I use it primarily for vocalists with my Sphere L22 mic from Townsend labs (which has an accompanying UA plugin) and DI'd guitars and bass. solo has one, duo has two - you are able to run more instances of the plugins on the duo than the solo.

The difference between the solo and duo is how many processors they put in their to run their plugins. I decided for the solo recently for a number of reasons, one of which was to avoid the itch to buy a bunch of their plugins all the time. Depends on what you're trying to do with it.

Universal audio apollo twin mkii solo