OK, I finally got around to testing the output of my factory "Premium" cassette deck.
The signal from the deck to the external amplifier is a LOW level signal. In other words, line level.
I checked it by connecting the output directly to a factory speaker and I got very, very low volume out of it. I had suspected this was the case from looking at schematics of the factory sound system. It shows two different connections for the deck depending upon whether or not the external amp was present.
I hope this clears up a lot of questions for people.
-Wes
98 SE Sport