No doubt I'll be corrected, and I don't know the actual origin chapter & verse, but basically any flat earth belief is that which is contrary to received 'scientific' evidence; which in this context, at the time in question(late 70s?), was that, for example, turntables could not affect the sound of a system, and that all amplifiers sounded basically the same. Linn and Naim set out to prove this wrong, so earning the label Flat Earth.
Everything else associated with Flat and Round Earth hifi followed - e.g. that Flat Earth systems exhibit pace, rhythm and timing, and that Round Earth ones have imaging, tonal neutrality, etc.
Hope this helps. The search facility didn't.
