Who makes the best passive volume control?

The Lightspeed is the best we have tried.

However all passives have certain limitations, and if you exceed those limitations, they will color the sound quite noticeably, and rob you of dynamic impact, regardless of how good they are.

The primary limitation is cable length. By the time the cable is even 6 feet long, the loss of bass and impact is easily heard. At full volume it will not be such a problem, but as you back off on the volume control, it will be very much like operating a sort of tone control that rolls off the bass. This is caused by an interaction between the output impedance of the source, the resistive value of the passive unit at the particular volume control setting, the characteristics of the cable and the input impedance of the amplifier. That's a lot of variables.

It is for this reason that you will see an on-going debate about whether or not active preamps can be more transparent and more dynamic, with greater bandwidth. Some active preamps are much better at this than others! In theory, the active line section should control the cable so that it has no artifact. In practice, this is rare although there are examples (most of them being balanced designs). However, if it does control the cable the passive will not hold a candle to it.

A little OT but how can you tell if an active is controlling the cable in the way you describe? Any good way of measuring this? In something like Stereophile's equipment measurement, any particular that will give agood idea of cable control, degree of cable control or likelihood of cable control?

This is easy to answer. Try using a cable that is 100 feet long. Then try a 1 meter cable. Obviously you will hear a difference, and it will also be something that can be measured. As you decrease the volume, you will find that artifacts are easier to measure. It will not matter how good the passive control is- it will matter more that it *is* a passive control.

Now I know that no-one uses 100 foot cables, but this model will allow you to see and hear the issue.

You could also use a very cheap cable and a well-reviewed cable. You might argue that the difference you are hearing is that of the cable, but of course that is the point.