What’s the best way to sample guitars or voices?
Because the MPC1000 accepts line-level signals and doesn’t include any kind of preamp, these kinds of signals work best with an external preamp. The Rec Gain control does allow you to amplify weaker signals, and the manual does talk about hooking up a microphone directly, but condenser microphones work best with a dedicated external preamp, or at least a decent mixer channel preamp. The same applies to guitars, but even more so: the whole debate about amps vs preamps vs modelling is too deep and wide to get into here. In short, the things you need to get a good guitar sound in the studio are the same things you need to sample a guitar properly – whatever those things are for you.