After more research I've found these comments:
http://www.mylespaul.com/forums/showpost.php?p=1378699&postcount=30
http://www.mylespaul.com/forums/showpost.php?p=2234477&postcount=37
http://www.marshallforum.com/index.php?threads/fun-with-lead-12s.10644/
It seems at first marshall did use the diodes in this amp, and Gibbons too... xP
http://www.mylespaul.com/forums/showpost.php?p=1378699&postcount=30
http://www.mylespaul.com/forums/showpost.php?p=2234477&postcount=37
http://www.marshallforum.com/index.php?threads/fun-with-lead-12s.10644/
It seems at first marshall did use the diodes in this amp, and Gibbons too... xP