Originally Posted by karlsaudio
You didn't answer my question.
OK, I'll try.
We're talking about a line level signal, here, which means we're dealing with voltage. When you split a line level signal into two inputs, as long as the input impedances are relatively high compared to the output impedance, there shouldn't be much of a (if any) voltage drop in the system. The sub's inputs do have a high impedance and the AVR's sub pre-out does have a relatively low impedance. As I said, the Y adapter doesn't just simply split the voltage in half. It doesn't work that way. Potentially, the same voltage signal can reach each of the sub's inputs as would reach a single input were only one input used. And since the sub sums the inputs, the result is a higher input signal to the sub's amp than would be there were only one input used, which results in an increase in output.
The same sort of thing happens when a Y adapter is used to split a line level signal into two separate subs, or into a sub and an amp, or into 2 separate amps. As long as the signal is feeding inputs with relatively high impedances, there is no appreciable signal loss. It is certainly not split in half. Ask anyone who uses multiple Y adapters to connect multiple subs if they think there is significant signal loss across the Y adapters.
The exact increase in output that results from using a Y adapter and both of a sub's inputs has been measured and reported here in these forums many, many times. It's real. It's just not a useful and/or usable increase.
Now, I may have boogered something up in that explanation. I'm not an EE. Maybe someone else can explain it more succinctly or in more understandable or simpler terms than that. You can also read, yourself, about how line level signals behave when they're split into high impedance inputs.
But that is about as much time as I want to waste on this.