side note I didn't get to this stage yet of my Rpi3b+ what are you guys doing for audio out from the Rpi3b+, any one sell super cheap 3.5mm audio with 6ft-12ft +/- so I can splice it and solder it into my speaker when the time comes. I don't anticipate on adding a sub likely just a single 6x9 speaker.
Look on ebay or amazon.
That jack will not power the speakers directly, you will need to use an amplifier to step up the current.
I think the simplest way is to buy a cheapm set of pc speakers and use the insides.
You will have the cable, jack amp, and speakers. Im not 100% sure that a regular audio cable will work in the RPi jack because that jack also has a connection for video.
You can also get a cheap USB sound card, I think it sounds much better than the 3.5 mm jack on the RPi.
ajmartin: can you post your config.txt file?