Getting audio signal from IKEA Symfonisk (SONOS)

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I would like to get a clean audio signal from my IKEA Symfonisk in order to connect my speakers to my SONOS network. I basically want a $99 SONOS AMP with mono output to ceiling speakers.

I started looking into doing this after seeing this teardown video on Youtube:

I found this guide where the author connected a RCA connector to the audio signal from the DAC chip (PCM5101A):

I tried this method and was surprised to hear a superb sound coming out of my speakers, until the system tried to deliver som bass. The sound gets muffled when there is some bass in the the track. I have two theories of why I am not getting a clear audio signal, theory 1 is that it has something to do with the mixing of the two audio channels. As the author states in his guide:

The Symphonisk uses two unbalanced outputs from the DAC. One for the low frequency driver and one for the high/mid driver. We will combine these outputs, relying on the existing resistors to create a passive summing mixer circuit. The software crossover will be adjusted by the Truplay tuning to fix the frequency response to match the new speakers.

My other theory is that when soldering the connector into the circuit, you probably should cut off the rest of circuit from that point on so the Symfonisk PCB doesn't try to use the signal at the same time that the RCA connector is trying to send the same signal to an external amplifier.

My questions that I need some help answering is:

- Should the passive summing work, or are there a better way I can sum the signals?

- Should I try to desolder the resistors on the analog signals coming out of PCM5101A in order to prevent the signal from being shared with the rest of the circuit? Do you think this will make a huge difference?

- Would it be possible to use a device like this and connect to the I2S digital audio bus in order to get an analog signal? audio - How to make PCM5102 DAC work on Raspberry Pi ZeroW? - Raspberry Pi Stack Exchange
Would the external device and Symfonisk circuit be able to decode the I2S signal with a parallel connection to the same bus?

The DAC is this one:

Thanks for any help! I will make sure to give back by writing a guide on how to do it if I can make it work!
I was just about to try out the speaker with only one channel at a time like you suggested, but thought I should try to just plug in headphones first to hear what it sounds like. I played the same tracks with two identical headphones, one connected to the Symfonisk DIY line out and the other to my iPad. The bass sounds just fine through the headphones, I can even tell that the trueplay process seems to have set the Symfonisk in a state where it overcompensates for the lower frequencies. I will try some other speaker tomorrow, until then I am open for suggestions for things to try out.
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