Recommendations required for download managers

Any good recommendations for download managers that can dynamically change the speed for one amongst multiple downloads ?

Say I have 3 downloads, each going at 5.6, 4.3 and 3.2 kBps respectively (maybe from same or different servers). Now, I'd like to raise the speed of the 3.2KBps to say 5.0KBps, but don't mind the other downloads dropping to a lesser speed.

Is that possible ? Have you seen/used it ?

Any recommendations ?