[clintj]

I'll take the negative feedback question.

Negative feedback (NFB) takes part of the signal being sent to the speaker and returns it to an earlier part of the amp, out of phase. This lowers the gain of the power section just a few decibels.

Side effects of NFB are lower background noise, flatter frequency response, and the amp becomes more resistant to breakup/distortion. When you finally push the amp hard enough, though, the NFB effects go away which causes the amp to break into distortion more abruptly, more of an on/off feeling.

Blackface Fenders have a generous amount of feedback built in, which helps them maximize headroom (like in the Super and Twin Reverb). The tweed Deluxe and Vox AC30 have none, so when they hit overdrive it's a gradual transition. There's a broad range available from just slight warmth to full out roar, and the player's picking and guitar volume controls where in that range he lies.

My opinion is that amps with lower gain are more sensitive to touch. Higher gain amps like Mesas, pretty much any touch of the strings gets you the same response on the lead channel.