Linking amplifier to sound organ causes fault
According to the linked page, it's designed to operate from its own microphone, thus providing isolation  from the audio source via the air. Connecting it to anything else directly is potentially dangerous. (Even if the polarity is correct, it may connect "live" to your precious stereo should one of its components fail short-circuit...)

To connect it to any other device you need to provide isolation : the simplest way is via an audio isolating transformer. That one is fairly expensive, designed for professional audio, but its flying leads make it easy to connect up. This one is probably adequate for your needs if you can make a small PCB to connect it up.