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Lots to choose from out there…
I've found the integration between ProTools and Structure very smooth; it's easy to wrap my head around and actually produce some decent sounding patches in a short amount of time, which is really important to me since my schedules often don't allow for as much pre-build design as I would like.
Structure is a lot of fun, and is simple enough to navigate. Much easier, I think, than SampleTank. I didn't find it very sound design-friendly, as far as loading my own samples from recordings, but it was simple enough loading 3rd party libraries. Although not as famous (infamous?) as other programs, I had fun using 112dB's Morgana. Much easier to navigate than SampleTank, in my opinion, and certainly less trance/house looking. What I really want to try, though, is MachFive. There's no demo available, though, so I suppose I'll have to save up for it.
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