Only the rec volume controls how loud your recording will be, the main volume is just to control how loud the sound comes out of your speakers. Your problem can be that if your sound source is a turntable, you must be sure that the incoming signal is at "line level" (the signal that comes out of your turntable is at "phono level" and needs to be pre-amplified by using the pre-amp of your mixer for example).
If I'm not mistaken (I'm not in front of my MPC), after sampling the sound and when you moved to the trim screen => push Edit => push Open Window. There you can zoom in/out on your wave form to set for example the start/end points precisely. Otherwise, you could save the sample on disk and load it into your PC/Mac, and edit it with a wave editor program.