Don’t forget that adding the parallel resistor also reduces the open-loop gain of the output stage to the point where the feedback loop no longer works, which results in more drive from the phase inverter, causing earlier clipping and more distortion (which can be a good thing, if you are trying for more distortion at lower volumes, but it does change the tone and feel relative to the full-power output). In addition, when the feedback loop is broken, the presence control no longer works.
Also, adding a low parallel resistance across the primary of the OT makes the amp’s output source impedance look very low and very resistive, which changes the damping factor and flattens the frequency response, eliminating the natural bass and treble boost you get from the speaker’s impedance curve. The end result is a flatter, midrangey tone.
Consequently this means if you’re looking to acheive that tone from Van Halen 1, and aren’t using this (potentially harmful) method (and are using something like a PPIMV instead), then it would be a wise idea to disconnect the negative feedback loop in your Marshall. This would be a 27k, 47k or 100k resistor that connects to either the 4/8/16 ohm taps for the ohmage selector switch, or to the speaker jack.