Andrew, if you're still watching this thread I thought of something that might be nice for people to see.
Since you have different DSP routines for the different speakers, can you make a "null" routine? One that just passes the input to the output?
Load that, and post a sweep showing how transparent the ADC-DSP-DAC signal chain is in the audible frequency range.
As for a direct digital input, the above should show how unnecessary it is, but also it would be difficult to make use of it. First you'd need something doing digital volume control, unless you like ALL REFERENCE LEVELS ALL THE TIME. Plus with HDMI and HDCP it's difficult to get a digital output for most media these days.
