Both ways have been mentioned:
1) Use an intermediate device to decode the HDMI audio, send it to speakers including the sub. (Most people use a receiver on an existing setup. Hubono mentioned he uses his soundbar.)
Advantage: You can use big speakers / better sound.
Disadvantage: More remote controls & power required.
2) Buy a sub that sits between the amp (Q) and the speakers. These have jacks for connecting the speakers to the sub, and then connecting the speaker wire to the Q. They will look at the speaker signals to create the music. See below for an example back panel:
Advantage: No remotes!
Disadvantage: Not all entry level subs turn off with no sound.
I have a pair of Polk Monitor 30s on my Q. While sounding good, they in combination with the Q definitely lacked bass for music. I moved my PSW10 inline (option 2), and now music sounds much better.
GNex i9250: Ver: 4.2.2; Kernel: 3.0.31-g9f818de; Baseband: I9250XXLJ1; Build: JDQ39
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