The first people do (myself included) is to check what's inside (if that is possible). It can sometimes be good to know what's inside since (as you know) some brands of semiconductors have a good reputation.
The Duet doesn't pretend to be ultimate high end. Heck look at the I/V converter - an LM833. The Duet is full of them. But they do very well with the components - the real star is the 6 layer PCB. (Something you, if anyone, would appreciate heh )
The reviewers do often just listen and (luckely) they don't know anything about electronics. LM833 isn't particulary well regarded in the DIY circles at least but I'll guess it's quite alright. I wonder how a LME49710 (quite cheap, 2 USD) would sound?
In Sweden the Duet got very got reviews in the magazine MusikerMagasinet.