The idea of a 100+ item dropdown menu scares me. And if you're going for multiple selections, then checkboxes are your best bet.
But a 100+ item checkbox array is overwhelming, too. I'd suggest grouping similar channels into relevant sections in order to take advantage of clustering and make the menu more usable.
Also, I like the idea of having little icons for channels, if it's viable. It could lead to a very cluttered, icon-ripe layout, but it's worth a shot. If you like the idea of grouping channels together, maybe have an icon for each section.