I made a new icon for ZynAddSubFX, with a much more elegant look. In the Mediafire folder you can find all the essential sizes and two SVG files: One for Inkscape, and one Plain SVG. Attached to this post is the 256px icon.
http://www.mediafire.com/?578wvfn4p5oet
256px icon
The icon looks nice, but the lack of color strikes me as generic.
Any reason you are using shades of gray rather than more vibrant colors in the design?
Here's a variant based on MinkoSoft's design, cleaned up a little, and colourised using a garish palette based loosely on the current dark theme (blues and greens from the frequency/waveform display in the addsynth voice editor).
It looks decent at most sizes. The colour scheme is intense, but not too off-putting just for an icon, and the logo is highly recognisable.
I also tried a variant with a slight gradient on it, but I didn't like it as much.