I really like the Horse Magazine for info on “the staples”: http://www.horsemagazine.com/thm/great-stallions/
The other source that I find great… you are on one right now! Search you desired stallions name in NA and German breed forums and you will find golden info that will not be in any stallion brochure; semen quality, true offspring temperment, conformation flaws, stallion owner reliability etc. Some posters must be taken with a grain of salt but you can get a good overall picture.
Also watch the non-manufactured video clips of them posted on youtube, generally taken by people in the stands at stallion shows. I find these unedited videos very helpful.
Example: I am interested in a bigger named younger stallion - from watching unedited clips of him on youtube at a stallion show I realized he is in heartbar shoes at the age of 6. Clearly there could be alot of reasons why this is so but that info provides me with valuable knowledge that the mare one might use needs to have a good set of feet.