They are miked a few inches away from the source but central to capture the overall sound from the amplifier, there is no particular distance from the source the microphones should be as it is personal preference.
The MS microphone technique is one of the most complex techniques and took me a few times of going over and learning it before I managed to set up the MS matrix successfully.
The MS technique uses 1 figure of eight microphone and one particularly cardioid. So I chose to use two AKG 414’s condensers and set one to figure of eight and the other to cardioid. The microphones are set up so the cardioid microphone is facing the sound source whereas the figure of 8 microphone is coincident at 90 degrees, inverted over the other so the same distance is shared resulting in them being 180 degrees out of phase.
The MS Matrix to set up this technique involves 3 mono aux inputs, the cardioid being the MID microphone and 2 inputs for the side microphone (fig of 8), one SIDE positive and the other side negative, panned maximum left and maximum right. The mid microphone uses input 1 whereas the side inputs use input 2, then a plug-in is inserted on all the channels that enables phase inversion on each aux input; so I used TRIM. The phase then needs to be delayed by 4 samples and the SIDE – is phase inverted. Then a mono audio and a stereo audio track are added, I then bus the outputs of the mono aux inputs to the inputs of the new audio tracks. So the MID to Bus 1 (mono) and the SIDE – and SIDE + to bus 3-4 (stereo). A master fader is then added and Phasescope, if the MS matrix is set up correctly there should be a horizontal line showing that they are 180 degrees out of phase when you highlight the SIDE channels and a vertical line of 180 degrees when highlighting the MID channels.