I had previously stopped using s-04 as all my beers brewed with it had a distinct yeasty-tang that as you said, got stronger as it aged. I even entered one of the beers into a comp (it was a special bitter) to see what the judges thought. The beer scored a 33 and one of the judges commented that the beer tasted like I had used s04 and fermented it too high. He was right.
Now whenever I use s04 I ferment it as low as it will tolerate. No more off flavors.
Bierhaus! where have you been? update your blog please.
but on topic, what is the lowest you've had a decent ferment with S04? It struggles in the 16C range and often flocs out early. It seems to have a very narrow temp range that makes decent beer (17-19C).