Interesting, the copper chiller I use is basically a miniaturized version of that cascade chiller. Does the beer flow through the pipes or down over them? Mine has only 4 tiers so it isn't very efficient and I have to recirculate the hot wort for about 45 minutes to get it down to pitching temps. The extended recirculating is what makes me suspect aeration and copper exposure as a potential source of the issues I'm having.This old german cascade chiller looks like copper to me:
http://www.braukaiser.com/wiki/index.php?title=Museumsbrauerei_Schmitt
On a side note, in my setup the hot wort flows from top to bottom while the cooling water flows the opposite direction to maximize the temperature differential at the point of exit.