The client's hi-fi system had the most spectacular bass I'd ever heard in a living room. Rich, deep, room-filling low end that you could feel in your chest. Then his wife shouted something from the kitchen and he hit the mono button on the preamp. The bass vanished. Not reduced. Not diminished. Vanished, like a magician's assistant through a trap door.

In stereo: thunderous low end that rattled the picture frames. In mono: a thin, reedy sound that a transistor radio would be embarrassed by. The difference was so dramatic that the client's first theory was that the mono button was broken. His second theory was that stereo 'creates bass somehow.' Neither was correct.

One speaker was wired with reversed polarity. In stereo, separated by three meters, the out-of-phase bass frequencies partially reinforced each other due to the path length differences and room interactions. It sounded big, but it was an accident. When summed to mono — electrically combining the left and right signals — the inverted polarity caused perfect cancellation below about 200 Hz. The bass was subtracting from itself.

I swapped two wires on one speaker terminal. The stereo bass reduced slightly — the accidental room interaction was gone — but the mono bass was now solid, even, and present. 'It sounds worse,' said the client, who had been listening to phase cancellation artifacts for four years and calling it 'warmth.'

The Moral: Always check mono compatibility. SonaVyx's transfer function with polarity verification would have caught this in seconds — beautiful stereo bass that disappears in mono is a polarity error, not a feature.