"The Differential Reference Module in the XPA-DR1 is made up of a perfectly symmetrical balanced pair of high-performance amplifier modules. Each individual amplifier module already has vanishingly low distortion but, by connecting them as a balanced differential pair, a significant portion of the tiny amount of remaining nonlinearity is cancelled out. As a result, we are able to achieve even lower levels of distortion in the XPA-DR1, resulting in remarkably transparent sound, with unprecedented smoothness and clarity."
So it uses a pair of amplifiers that output one signal I think. The problem lies in the polarity possibly being reversed I think, and that doesn't match well with a powered speaker.
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