Thanks for helping. I've already tried putting the blur all the way down (and fiddling with the noise, no idea what it does either) - the screenshots in my post are both with blur at 0 :( Also means my logout screen isn't blurred properly and the icons are harder to see over the background.
My panel opacity is set to "Adaptive" so that it turns opaque with maximised windows. Even if I set it to be "Translucent", it's looks the same as it is in my screenshots.