'Flat earth' usually has something to do with a lack of math coupled with ignorance. Here are the issues of what a passive has to face:
With any passive volume control the control element is a series element with the source. The more you have to turn the volume down, the greater the source/volume control impedance becomes.
Now most digital products make about 4 volts output, so you do have to knock the signal down in order to use it.
So if the source impedance is 1K, to listen to something you might have the control turned down so only 200mV is coming out of the source. With an amplifier with 25 db of gain (most amps have 30) this will result in about a watt and a half from the amp (nice for low level listening). But this might mean also that the source impedance is now well over 10 or even 50 times higher, depending on the control!
You can easily see here that the cable between the control and the amp has suddenly gotten really critical.
One way to get around this problem is to keep the value of the PVC fairly low- 10K is about the limit though, and a lot of sources will be unhappy with that, as the higher the volume setting, the more the control is in parallel with the input of the amp. If the amp also has a 10K input impedance your source might be asked to drive 5K at full volume. This could kill the bass in many systems.
This problem is solved in an active preamp because the volume control is buffered from the load of the interconnect cable and the power amplifier input by some sort of active circuit.
A passive of course has no such circuit, so now the increased output impedance of the volume control and source exacts a price upon the sound, and this will be that the capacitance of the cable is able to roll off high frequencies at a lower frequency (introducing phase shift, which messes with the soundstage presentation), and if the source has an output coupling cap, we will see a low frequency loss as well.
(Resistors and caps are often the elements of filters. To roll off highs a resistance is often followed by a cap to ground; the example of the PVC turned down above is an example. If you want to roll off bass, put a resistance in series with a coupling cap.)
Note here that *it does not matter the quality* of the control! What matters far more is the *value* of the control. This is not a quality issue so much as a math issue. Quality is something you can mess with but the math is pretty hard to get around, sort of like trying to break the laws of physics.
So when you have a passive control you have to be very careful- for example if the amplifier input impedance is high you will have greater high frequency rolloff due to the cable. If the amplifier input impedance is low (10K for example) bass will be a problem at all positions of the control except full volume.
The only real solution is to place the passive control at the input of the amp. This could be really inconvenient (especially with monoblocks), unless the setup was also remote-controlled. Otherwise the use of a passive is always going to be a compromise. Anyone who has a successful setup will probably also have had to pay special attention to the interconnect cable at the very least, unless they had a lot of good luck, good advice or both. One thing is certain, PVCs are not a universal panacea!
As I pointed out in a post above, one of the functions of a preamp is to control the interconnect cable. This is done by having a very low output impedance. When you *raise* the output impedance, as all PVCs must do, instead of controlling the cable, you exacerbate its effects due to the math involved.
