Have you tried working back down one by one after he first shows signs he feels the stim? I recall Lou saying that once they feel it at a higher level they may be sensitize to it at a lower level. I would try that. Also have you joined Lou's forum? He's very helpful and responds pretty quickly to questions.
My dog works around 20 on the dogtra 1900 without distractions and up into the 30s with some distractions, but I wouldn't call him super tough.
Truman, GSD 3/3/2014
Cosmos, Super Mutt with a curl tail 7/2015
Havoc, Golden Retriever/Rottweiler 10/6/2001 - 11/21/2014