"something in the frequency range you've setup for the sub" can you elaborate this? Adjustment of cross over frequency?
Yes, your sub pre-out sends a limited bandwith signal, determined by your crossover settings, in combination with the circuit in the sub's amp, to switch it to on from standby.
The usual complaint is the sub not turning on because the signal level from the avr may be set too low, resulting in insufficient voltage to trigger the sub (and the gain on the sub too high); the solution to this is to lower the gain on the sub and increase the sub channel output level.
In your case the voices in a news show apparently didn't have enough content in the bass frequencies needed, the level seems fine. Does make me wonder, what is your sub's gain set at and that of your avr's sub channel level?