On the recent Get Lucky tour I became aware of a little technical detail I had never paid attention to before: I was sure that the guitar cable coming from the guitar would first go into the Ernie Ball volume pedal and then back to the area on the right side behind the stage where Mark Knopfler’s effect rack and amps are placed. Instead, I observed that a long cable leads directly from the guitar to where the amps are. In other words, the input amp of the effect rack (or some other) seems to be the first part in the signal chain. Here the signal is boosted to line level, then (before or after the other effects in the rack) it seems to go back to the volume pedal in the front area on stage, and back to the effect rack again from where it runs to the amps.
I always wondered why Mark’s volume pedal behaves different than the Ernie Ball pedals I have tried out (the response curve seems to be totally different, see here for more). Maybe Mark’s pedal really has a different poti (like some rumours say), or it just reacts differently because it works on a much higher signal level (after the first gain stage in the effect rack the signal becomes “active”, which means more level and a lower impedance). Theoretically it is also possible that Mark uses the version for line levels of the Ernie Ball pedal, which has a 25 kOhms poti instead of the 250 kOhms of the standard version.
I tried my pedal with a low impedance signal to see if it makes any difference: it did not, but I could not test with a higher level yet.
Another consequence of this signal path is that the long first guitar cable surely changes the colour of the tone. The longer a cable is, the higher its capacity which means it moves down the position of the pick-ups resonance frequency peak. It acts like a small capacitor put into the circuit. This way the cables capacity can easily shift the resonance peak of a standard Fender pickup (normally at about 6 kHz) to something like 3 or 4 kHz, which means that glassy high end at 6 kHz is reduced and the “presence” frequencies at 3-4 kHz are boosted.