TS, doesn't take that much figuring; when you feed both sub inputs with voltage by using the Y-adaptor, the two voltages are combined in the sub amp and increase the output of the amp at the same setting. This isn't a significant benefit since the the same result is obtained by raising the sub level control on the receiver a small amount to increase the voltage output, and nearly all HT receivers have plenty of sub voltage available.

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Enjoy the music, not the equipment.