I can think of a couple of possibilities... which could be both interesting and confusing.
(I can tell you that the XDA-3 doesn't "know" or "care" which of its outputs you connect... but subs can be more interesting in that regard.)
1.
On most subs the two RCA inputs are simply summed.
On others there is a separate LFE input... which may or may not act differently.
And, on some with a stereo input, you are told to use a certain one if you only use one (it may or may not actually matter).
2.
On some subs, including a few of our big models, the inputs are "intelligent".
So, for example, if the sub senses that you've sent signal through one input,
IT DISABLES THE OTHER INPUTS.
And, once that happens, until you power the sub fully off, it ignores those other inputs.
So, for example, as soon as it hears a signal on its balanced inputs, the unbalanced inputs stop working, until you power cycle it.
(And, if you simply send an audio input to the other inputs after that, it will be ignored.)
This can drive you crazy when trying to troubleshoot something... or if you want to use both inputs.
3.
It's also quite possible that some subs simply know which inputs are connected and change their behavior depending on which is connected.
They may do this electrically, or mechanically, with a little "feeler" inside the plug that actuates a mechanical switch.
I have a cool set of computer speakers that sense which inputs and outputs you have connected...
So, for example, they "know" when you connect a sub to their sub output, and enable both their subwoofer output and the high-pass filter on the main drivers.
This is cool,
except if you plug a wire into that connector, and connect the other end to a sub that isn't on.
If you do that, the speaker
thinks that there's a sub connected, so all the bass from the main speakers
stops...
But, since there's really no working sub connected, you just get no bass... and wonder why all the bass went away.
(And there's no setting or configuration option... it just "feels" when you plug a wire into its subwoofer output.)
So... in your specific case...
It's not impossible that your sub "knew that there was
an input connected"... but "was
only listening to the other input".
(Just a guess... but it wouldn't surprise me.)
Hmmm, I think I see what you are saying.
I'll be darned..
Unplugging the non-LFE RCS... sub is still rumbling 🤔
That sucker wasn't working until I connected two RCA's...
It's healed! 🙏
