Entwistle's sound, from what I've read, is mostly from bi-amping where he uses very clean subs, and overdrives the mids and highs (naturally overdriven tube amp) into separate cabs.
That, and he uses very new, bright roundwounds with very low action. He doesn't play as hard as you would think. In fact, in an interview he offered up that the real key was playing light and letting the amp do all the work.
But the combination of clean/distorted biamping, bright strings, low action will get you closer than effects.
I have managed to get close to that tone by simply overdriving my rig, using bright strings, low action and playing kind of a slap style with my fingers, "banging" the strings against the fretboard, right up around the last frets of the neck to get that "clanky" sound.