Be aware John (RF) is on vacation til next week and out of town.
instability in the high frequency area is what separates the men from the boys. In order to get really good hf phase stability, you're talking about professional/mastering type decks, most consumer decks are made with stamped parts that are not stiff enough, allowing the tape to skew during its travels. This includes the deck top plate. Decks made with cast parts and precision aligned give you the phase stability we have all become accustomed to from digital land.
Can't agree more about visiting Dale Manquen's site to get great background material on understanding tape recorder mechanics. And don't forget Jay McKnights site at MRL, technical reading material. Those are two of the people who helped design and build these decks in the first place.
http://home.comcast.net/~mrltapes/#TechPapersAlan Garren