I'm not familiar with Dopus, so I don't know whether Konqueror would suit your needs, but there is a small add-on
for Konq that enables a quick filter like you describe. Dunno if it's powerful enough, though. The standard Konqueror-addons package comes with a filter for file types, but it's kind of clunky to use.
I haven't regularly used a file manager in (literally) years; I'm very much a command-line guy. I started using KDE a lot recently, and have found that I really like Konqueror. I've started using it instead of Firefox for my web browsing, and have been using its filemanager interface frequently.
One of my favorite features (though it currently involves too many mouse-clicks, and closes all your tabs when you use it) is view profiles. You can configure Konqueror how you like (with multiple panes, tree/icon/detail views, toolbars, etc.) and create your own "view profiles", sort of like a visual configuration for each purpose. The defaults include profiles for web browsing, file management, Midnight Commander-style, and others. Then, you can just switch view profiles to start working with files instead of web browsing. No need to start up a different program. Since browsing and file management are the two major tasks I do on a computer, it suits me well.
Another thing I like is the web shortcuts; you can put in "imdb:Juliet Landau" in your Location bar to search the Internet Movie Database
for Juliet Landau (useful for those spur-of-the-moment "What was that movie she was in?" searches), or "gg:Linux" to Google for Linux. It's very handy, once you learn some of the shortcuts.
Konqueror does the browser/filemanager integration very well, I think. It has some room for improvement, but it's definitely on par with, if not better than, the most recent versions of Internet Explorer. But if you want a dedicated file manager, I'm afraid I don't have one to recommend.
Good luck in your search!