Did a bit of reading about DirectShow and I now more or less get what's going on there. File Source (URL) is a pull source filter (like all source filters?!?) since it implements IAsyncReader. Push filters implement IMemInputPin. So far it shouldn't be a problem to implement a monitoring filter (or filter wrapper), but there's still a much bigger problem: this solution would only work with sources implementing IAMOpenProgress, which WM ASF Reader doesn't (don't know about the MediaPortal File Reader)... that would lead to an inconsistent behaviour...