to volume pot or not to volume pot

Simple volume controls can be a significant cause of distortion. It all depends on the circuit and the components. The worst, and most usual type, is a cheap wire-wound pot, inserted on the hot side of the input. Such a control has balance problems, inductance problems, resistance problems, etc. Wiring it on the ground side and using a fixed resister on the hot side improves things considerably. Other modifications and innovations are numerous. How a volume control circuit is executed makes all the difference.

It controls the impedance of the signal running through the cable and keeps it stable. Passive preamps are notorious for sounding thin in the bass and mid bass because they can't control these impedance fluctuations.

Uh, sort of. The impedance of cables and input impedance of an amp is a fixed value. It is not fluctuating and it is not controlled. For the record passives sound thin when they are used with the wrong cables and amps. Mated with the right stuff they sound glorious. Blaming a passive for thin sound is like using salt in a recipe that called for sugar and concluding that salt is an inherently poor ingredient. I'm sure the EAR will work very well in some systems and not so well in others just like any other component in a system.

That's all I've got to say since you can search the archives and find endless debate on the topic.

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A couple of things... As was said earlier, if you completely open the volume pot you effectively take it out of the circuit. An audible difference between a two identical circuits, except with one having an open volume pot, would be highly unlikely. My system sounds much better running the EAR 834p through a pre-amp, but systems will vary in that regard. One other nice thing... if you ever need to have your pre-amp repaired or you sell your pre-amp, you can use the EAR 834p with the volume control while you are without a pre-amp. Kind of handy.

Cheers