i'd upgrade the duet if possible and get some better converters/clock. the duet does sound really good for the price, but does it make sense to spend so much on preamps if you don't have some pretty good converters?
i recorded everything on my first record with a presonus 80 dollar tube pre into a firewire 410 -- you make do with what you have
i did lose sound going into the 410 though as i wasn't able to bypass the pre's.
i'd check out somethin' like an apogee ensemble, and then a good single channel of pre, and an sm7b. that will get you a much cleaner, warmer sound and be a huge step up.
that all being said, the duet sounds good... recorded all the instruments on our last release with a duet. even for the drums. 57 on the overhead, d112 on the kick. used an re20 into the duet for all the vocals -- looking forward to start tracking with some better converters and preamps though... man, renting some time in a studio with better apogee stuff, working on a better clock and converter, is making the new sounds and recordings so much fatter, clearer, with more depth, etc.
hope this helps