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I don't know enough about sampling to know exactly what is making the difference, but by comparison, the Spitfires sound almost like they have a synth layer added in - some extra or imprecise component of the sound that detracts from the naturalism - and if you take away all the reverb its really obvious, especially on horns. The Spitfire stuff can sound good, but when you A/B it with the Ark stuff the difference really is immediately apparent on most patches.
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FWIW I have Spitfire's Studio Brass and Studio Strings (I used to have others that I've lost), Metropolis Arks 1, 2 and 4, and I've had Best Service Orchestra complete, Chris Hein's horn stuff, some NI orchestral stuff and IK's Miroslav, and in terms of naturalistic sounds, I think the Arks are unquestionably the best of that group.
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