Bump.. I don't have a DUO..
I'm thinking the Quad / Octa are similar in quality, except in the number of inputs - my main reason I've got my Quad, to test if I should buy the Octa.
I'm also thinking my Quad is more like a 'duo' now as I've never use the other 2 input/ouput via coaxial.
Here is my Quad capture performace :
@ 44100 sampling
48 samples / round trip 6.5 msec
64 samples / round trip 7.6 msec
96 samples / round trip 9.8 msec
I'm usually at 96 samples working with soft synths (it's fine too at 64 samples).