Yes, it isn't a major advantage(contrary to the implication in the B&W manual)to get a Y-adaptor and feed both the L & R inputs. The 6dB increase mentioned does occur, but this is only an increase in the voltage supplied to the output section of the sub amplifier, and doesn't mean that the maximum output capability of the sub has been increased by 6dB, or by any amount. The same voltage increase can be had by turning the sub output on the receiver higher, and most receivers are capable of supplying more than enough voltage to keep the sub amp "awake".
Some of this seems to be contrary to common sense, but the technology is clear. First, when a Y-adaptor is used the voltage on each output of the Y isn't cut in half, but is the same as the input voltage. A discussion and proof of this can be studied
here . Then, merely rejoining the two wires wouldn't of course double the beginning voltage(wires are passive elements and there aren't any miracles in audio), but the two voltages are fed into a summing circuit in the sub amplifier. This results in a doubling of the voltage, which is a 6dB increase(voltage doubled is 6dB, rather than 3dB when power is doubled, using the formula 20xlog[V2/V1]). An illustration and proof of the technology of a summing amplifier can be studied
here . So, this is true but not a big deal, and using one input is fine.
