Aiken Amps - This site has some great technical info on it. My favorite is Randall's explanation of why the Vox AC30 is NOT a class A amplifier. This is a widespread misnomer.

Ampage - This is a wonderful forum with lots of helpful members and a wealth of knowledge.

Duncan's Amp Pages - This is a cool site with some neat little apps for the amplifier designer.

The Valve Wizard - I love this website. Merlin is a really great writer on the theory of tubes and circuits. Aside from this great website he has written a book called Designing Tube Preamps for Guitar and Bass which I highly recommend.

Geofex - An excellent site geared mostly towards guitar effects pedals. There is a ton of great info and practical circuits on this site.

Free Info Society - A great source for schematics.

Q & A

Should I replace my 2-prong power cord with a 3-prong power cord and why?

YES. A 3-prong power cord is a great way to reduce the risk of death from electrocution.  I'm serious, it sounds funny, but really lets be smart here.  Playing an amplifier with a 2-prong power cord is like playing Russian Roulette.  If a high voltage wire happens to touch the chassis of the amp while you're playing guitar and touching a microphone you COULD die.