This is a great hack, however I really prefer the easier version of simply connecting the symfonisk board to a decent/great bi-amp speaker.

See, in their normal state these Sonos speakers are mono, yet they have this stereo amp in them. What Sonos does is use a crossover with that stereo amp to make it a mono bi-amp (in other works instead of left/right, it's outputting high/low to the tweeter and woofer respectively). If you simply connect one side of the amp to a normal speaker, you'll either get muddy bass or incomprehensible chirping, because the crossover splits the audio)

The whole soldering part of the mod here is to work around that fact, and bridge the audio back to a single mono channel. While this has some advantages - more power, and connects to 'regular' speakers, it's a lot more wrk, and a lot riskier.