I don't use Tivo Desktop Plus, so I don't know if it automatically converts downloads. If it does, there has to be a way to turn it off.
The easiest thing to check is the file sizes.
On the Tivo, the program info screen should give you a file size and you can compare that with the file size on the computer.
If they are close (within a few MB), then you have the highest quality possible.
The Man Prayer: I'm a man ...... I can change ...... If I have to ...... I guess.