Thanks for a terrific news reader! While I am new to QuiteRSS, I used NewsCrawler for a decade, and began writing RSS/XML code for a nonprofit newsfeed before blog software came along and automated it. Here is a suggestion based on my experience.
When "ALL" feeds are selected for a filter, sustain this setting even when new feeds are added later. This could be a default, or a radio button setting in either the Filter edit screen or the main Options screen.
As it exists now, when new feeds are added users must edit all filters and reselect "ALL", otherwise the new feeds won't be filtered. The reason being that users are presumably adding new feeds to supply their filters with more news, not less.
If this is possible now, I'd like to learn the trick.