Different grades of Optical cables?

I recently changed my amp from a standard stereo analog amp to a stereo amp with digital inputs (as well as the usual analog line inputs).

I had been running the audio from my DVD player as analog stereo, but decided to take advantage of the digital inputs and switched to an optical cable (PCM stereo). The difference in both DVD and CD playback was quite pronounced - more than I expected - with much better channel separation and a definite jump in vocal clarity and treble 'crispness' (subjective term, I know!).

In fact, the CD playback from the DVD player (Panasonic) to my ears is almost as good as the analog playback from my Cambridge 640C; bit depressing in a way!

The cable I'm using is a well-made but inexpensive ($20) optical cable; is it worth trying other cables, such as glass-fiber ones etc, or is there less quality range in optical cables?