Use both, use conservative tone settings and observe proper gain structure (conservative, preamp doing the majority of voluming) and just let your ear tell you where to stop. I do this with my bass rig. I have a small Ampeg B1-RE I use with a Tech 21 RBI preamp. The Ampeg volume is set 1/4 gain, 1/2 master, slight bump on bass, pre set at 1/3 gain, 2/3 master, bump on bass and cut on mid and treble, Sounds great with tone adjusted on instrument, and I also use reverb and BBE sonic maximizer. As far as "needing", I always consider how I want something to sound, within reasonable, conservative settings, before settling for "needs"-based reasoning. Reminds me of my father asking me, "Do you really NEED all that stuff?" Same diff. Let your ears tell you where to draw the line. Don't go overboard...