Tricky... I tried using the MOTU to send identical signals to the Mackie and O2R--I mirrored Bank A (1-8) to the analog outs. I'm afraid I only have an unbalanced 1/4 6' hosa snake to connect them. Matching levels is my biggest problem, I either need a balanced snake to the Mackie or need to adjust a jumper in the MOTU. The Mackie was clearly at a disadvantage due to this fairly lame connect and the fact that the signal would be passing through the mackie preamps turned up 1/4, (though the tracks were originally from the O2R's preamps).
They were more different than I exspected. Describing the difference is difficult especially considering the non-exactness of my test (needs refined)
(To me...) The mackie had more pronounced mids and lows but less high end detail. There are things about the mackie sound I like, I may even try using the mackie as an eq for a submix, but I wouldn't want to mix classical music on it.
The O2R can sound bad. It does not do that well with bright microphones, any harshness is not hidden. So you need eq... but the O2R's build in eq's are not world class. They're okay for shelving...
Anyway, to sound good, you definantly need some analog sounding stuff mixed in liberally.
Because they sounded so different, I really think I have not answered anything about buss summing. I don't have a way to fairly make the test. There are analog and digital boards I'd take over the O2R, but they wouldn't cost less. I need a better analog board to compare too... and better speakers, meters, ears, brain ect.
I learn a lot here... Thanks all.