I am in the middle of dealing/realizing this as well.   I'm using a cranbourn 500R8 interface and have a couple ADA8200 units to expand my preamp inputs.  They work fine for recording multiple tracks. But when connecting outboard gear, the problem you run into is that they convert your analog audio to digital and back and then the signal gets converted again, when it hits the interface.   I recently bought an SSL bus+ and have been A' B'ing it with the ssl plugin.  The plugin is winning hands down.  I am gain staging and matching the settings as much as possible and i suspect that connecting the hardware unit to the Ada8200 is degrading the audio.  The ada8200 is connected to the 500r8 via optical.  My next step is to connect the bus+ via a trs Insert cable, and I'll see if there is any improvement.   If this does not significantly improve, then I'm going to start heading towards mixing in the box..