I used Download Manager Tweak for a while and found it to be worse than the default download manager.

What do you intend to use Download With to download with?

The reason I say these things is that I've been using DownThemAll! as a replacement for both of those extensions. While it advertises itself as being a way to quickly download everything on a page (and I guess you could use it for that), it works really well as a download manager. I mostly use it to download a series of large things one at a time. That is, download this one ISO, then download the next, etc., without having to interact with it. And it just plugs itself into the normal download dialog; it just becomes another option besides "Open" and the normal download.