Re: recommend an amp theory book
The Torres book is OK as a starting point for mods. Not entirely reliable, but written in language that non-techy people can understand, and you'll have a much better understanding of how amps work by the end of it.
If you want to get serious, Kevin O'Connor's The Ultimate Tone series is regarded to be the finest writing available.
I really like Aspen Pittman - The Tube Amp Book, if just for the vast range of schematics in there. Once you've got your head round how the various parts of the circuit work from the books above; it's fascinating to see how designers have taken different approaches to achieve what they saw as a great amp.