Which ever you chose just make sure it's capable of accepting line level balanced input for outboard pre amps. Many posts I've seen asking advice about outboard pre's without regard to wether or not it's compatible with the interface being used with it. Not a lot of interfaces have discrete line levels inputs that are separate from the onboard preamp circuits. IMHO if you've invested in outboard pre's I would want dedicated line inputs so as not to add unneeded coloration, noise, etc from the average pre's in most interfaces.
The Duets would work fine. I use a Motu that has separate line ins.
27" Imac 2.93 ghz I7 12GB ram OSX 10.6.5,
MacBookPro 2.4 ghz duo 4GB ram,
Mackie Onyx 1640I, Motu Ultralite Mk3,
LA610 Solo, Logic Pro 9.1.6 DAW