+1 for the NEETS materials.
And since you are comfortable with mathematics, you might want to take a peek at RCA's classic "Radiotron Designer's Handbook" which is free to download:
http://www.tubebooks.org/Books/intro_RDH4.pdfAlso check out Morgan Jones,
Valve Amplifiers and the texts by Martin Blencowe.
I'm still very much a beginner, but like you, I want to learn as much as I can about audio circuits and design. To that end, I find the following two texts that deal generally with electronics very helpful, especially with circuit theory:
Paul Sherz and Simon Monk,
Practical Electronics for InventorsPaul Horowitz and Winfield Hill,
The Art of Electronicscheers, derek