> input Z of the preamp? According to my calculations it ought to be about 501 ohms.
You (or I) must be misreading where the 15K feedback goes. As I read it, the 15K hits the 802 on the low side of the transformer winding. The high side is loaded with infinite ohms, some pFd, 8K of dead resistance, and 1300pFd||619 Zobel. Over most of the audio band, 8K. Reflected back to the primary at 1:2, about 2K.