I have had a quick skim through the thread and nobody has mentioned that the order of the multiplier and the extension tube are placed between the lens and camera make a big difference to the final image.
A quick and dirty example.
First up, a Canon 50mm F1.4 lens:
http://i17.photobucket.com/albums/b6...ms/05_50mm.jpg
Next up we have a 2x multiplier and the 50mm lens:
http://i17.photobucket.com/albums/b6...ms/04_2x50.jpg
Then a 25mm extension tube and the 50mm lens:
http://i17.photobucket.com/albums/b6.../03_ET50mm.jpg
Now we add the 2x multiplier
between the extension tube and the 50mm lens: (makes very little difference except for subject distance)
http://i17.photobucket.com/albums/b6...1_ET2x50mm.jpg
And finally, simply swapping the multiplier and the extension tube (Multiplier, Extension Tube, Lens) produces this result:
http://i17.photobucket.com/albums/b6...2_2xET50mm.jpg
Cheers