Yes, Adam, you'd be wise to take that $90 cable back and just thank them instead of buying it. As previous replies have indicated, there's no basic quality difference between the coaxial and optical connections. One possible explanation for your impression is that the output levels of the optical and coaxial outputs can be slightly different and the slightly louder one might have sounded "fuller" to you, but a small volume adjustment would have the same effect.

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Enjoy the music, not the equipment.