A good OD pedal will generally give you a greater range of tones than an individual amp is capable of. The other factor is that to get a true overdriven tone from a tube amp--i.e. saturating or "overdriving" the power tubes without artifically adding preamp gain--generally requires very substantial volume levels. Players who want to get this kind of tone at home or in a small room may find this virtually impossible without deafening themselves or family/neighbors. Good OD pedals will let you achieve a close simulation of this coveted tone at "bedroom" volume levels.