I get all the "tone" thing while practicing, but I really don't see how it can hurt practicing without amp, tone is important, but more important is to be able to hit the note you have to hit at the moment you need to hit it, when you practice without amp you are practicing scales, creating new bass lines, building strong fingers, getting faster, etc ... In the end tone generation has a lot to do with where on the bass to play and pickup selection, the rest is knob tweaking and chances are that what you are hearing in your amp at home is going to sound very different in a different space and with different gear. I would think that if you practice focusing on your tone you risk becoming gear dependent, and gear changes all the time but you can't change your hand or fingers. Jmo.