Okay Ken, the mystery is the M80 highs vs M3/M50 highs. Since you didn't mention it, I'll ask if you ran through the usual suspects: binding post connectors tight, listen to see if tweeters working, compare one M80 with the other to see if there was a difference, etc. The 525 wouldn't appear to have anything to do with it, assuming that you didn't have the tone controls or other sound processing in effect. Could it be that the stronger bass of the M80s simply made the highs sound subjectively less prominent?

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Enjoy the music, not the equipment.