I'll just chime in to specifically address your question and say that I think getting HDRLight Studio is probably the thing that might help you the most... I mean, it is designed specifically for quickly experimenting with lighting for product renders -- and that would be the thing that is probably the biggest issue I'm seeing.
Here is a version of your MXS file -- I adjusted the materials slightly, got rid of your lighting (I used HDRLight Studio to bake up a replacement environmental EXR, which is included in the zip file), and tweaked your camera... mostly I was using your photo (from a few posts back) as a guide, though the model you provided is much more simplistic than the ring in the photo (and that has a big impact).
Here is a version with a bit of simulens and levels/curves adjustement:
I am not a jewelry rendering pro (I don't even like jewelery much) -- nor even a product rendering guy. So the fact that I (who basically knows nothing) can get a somewhat respectable result in a few minutes of playing should be encouraging to you, who does already have that refined taste and eye for what would look best for your specialty market. Once you have HDRLight Studio you can play with the lighting endlessly until you have exactly the results you want... honestly everything else is pretty simple as far as Maxwell is concerned, and you already have a ton of good advice about that in this thread from seghier.