With those connections, I doubt that your ST50 can push a subwoofer at all. For best results, you'll need a receiver to connect the speakers to and then run the video to the ST50. Optical out is usually used when you are using the internal ATSC tuner of the tv and want to hear 5.1 audio from either OTA or the SmartApps if they support 5.1 out. ARC is another option but all that really does is save you from having to use an optical cable. However, the avr has to support ARC as well. IOW, both devices, the tv and the avr have to be ARC capable.