The Z-mode is where the color gets added. I would say in Mid-Z and set flat this preamp does not impart any color to the tone, but the pickups feel somewhat buffered and not so passive. Mind you it's the tone of your bass, but the interaction with the blend is much different than passive/brighter/clearer. High-Z adds zing and bite to the upper mids, low-z is where it is at if you need to cut through. The Low-Z needs tweaking with each set of pickups I tried, they all respond different for it. I almost always boosted the mids on this thing until the hex-screw was pointed at the z-switch. It sounded perfect right there, added tons of guts to my tone.
I have one audere that I am keeping, in a P-J setup and the Low-Z mode is awesome for that bass. the high-z sounds amazing with a P. The mid-z is very nice with the buffering/blend and a P-J. I never need to mid boost on that bass, its all there flat. The EQ adds to that bass, and is not needed for my sound. The z-mode is really the most useful thing on that bass.. that and the tone knob.
My reasons for choosing the east is for the boost only bass and treble cut point. The sweep mid is like deep frying something, everything is better deep-fried/swept-mid.
