Re: volume vs. gain--
My understanding is that gain regulates the signal before the final amplification--I think of it as boosting the input side. Volume regulates the amplified output. Each step in the process adds distortion, and amplifying distortion makes things worse. For the cleanest sound, you want to minimize gain. For some intentional distortion (popular in rock music) you want more gain. Having both gain and volume controls gives you some flexibility in the sound you produce.