Nilo,
I almost bought the Onyx...but I bought a lot of gear and bought the Alesis FW16 mixer instead (as it is a bout $1000 cheaper). I know the pres are not quite as good...but it has been surprisingly versatile and plays nicely with Logic. I also wanted something that could be used as a live mixer.
The Onyx FW Card does D/A conversion...you will not need a stand alone converter. Great choice...the Mackie should rock your project studio.
Cheers,
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Nelly Drummer, Vocalist, Project Studio Stunt Pilot
“My vocation is more in composition really than anything else - building up harmonies using the guitar, orchestrating the guitar like an army, a guitar army.” Jimmy Page