My old S4 connected to it fine, and other devices (iPods, computers) are working with it currently, but my S5 just isn't at all. The S5 is also fine with other auxiliary cables and headphones.
Hmm, not sure why it isn't working. Compare the 3.5 mm plug side-by-side with another aux cable that you know works on that phone--are the contact rings in the exact same position?
They are in the same position. I tried it with a different stereo and different phono cable and it still didn't work. I'm confused and pretty bummed. I really hope that my new phone will work with my main speakers!