I have been plagued with this issue for months and have found no result. When saving images (photos, logos, really anything with lots of colors) for the web, images show up washed out, muted. I have changed color profiles to sRGB, turned proof colors on, proof setup to monitor RGB. When saving, sRGB is checked. What am I missing? This is happening in both PS and AI, it is muted in preview mode and in live and non live browsers.
Gamma settings? What OS are you using?
What format are you using? If you are using gif you may want to try jpeg or png. since gif doesn't have as many colors as jpg or png, this could be problem.
I've tried recreating your problem on PS and I think the effect when using gif with 128 or less colors.
and as danwellman said. it would be good if you point us to an example.