The sub turns on when it gets an input signal. If the input signal is too weak to detect properly the sub won't turn on. So you can boost the input by turning up the sub level on the receiver. Though, as pointed out above, you'll need to turn down the volume on the sub to keep the over-all output level the same as before.
One thing I've noticed with my Sony sub: the turn-on level is higher than the minimum output level--it's possible for the sub to play a signal, once it is on, that wasn't loud enough to turn it on in the first place. That is, I can play some part of a passage without triggering the sub. But something louder comes along, and the sub now does power up. If I go back and play that previous part I will hear some output from the sub.
I probably should set my receiver somewhere between +1.5 and +3, but I fear if I'm ever listening higher than -10 dB that I might start clipping the output driver for the sub. (That fear is pretty unfounded though, as I don't listen any louder than -12 dB at the craziest of times, and if I was that hot, and there was LFE content which was mixed at -0 dB, I'd be diving for the remote, to cut things back).
