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I have a guitar amplifier which is rated at 120w (solid state). It has two outputs (A & B) both of 60W each.
If I were to run both of these outputs into a 2x12 cabinet in stereo (output A to stereo left and B to right) would I get 120W total power or would I just get 60W?
There should not be any significant difference if it is the same cabinet.
A typical Marshall 1960A cabinet comes with the options 4/16 Ohm mono or 2x8 Ohm stereo. If your amp supports bridged mode, you can output the combined 120 watt in mono mode, or you can simply use two cables in stereo mode. The output will be the same.
If you instead have two different cabinents - one in mono, the other in stereo - and the total impedance for both is the same, then other things matter:
In theory if we disregard physical speaker and cabinet differences (loss of energy, sensitivity, cabinet absorbance etc) and assume that the total impedance of the combined speakers is the same, then yes, the perceived loudness will be the same.
But in practice the answer is that each speaker alone comes with a loss - it takes some energy to drive the membrane, and the sensitity will vary from model to model. Also the actual construction of the cabinet (open/closed, materials etc) and the distance of the speakers will influence how much pressure you get in the end.
