“I believe the output from most sound cards is speaker level, designed for PC speakers and headphones”
Sound Card Line out
The signal out of line out remains at a constant level, regardless of the current setting of the volume control.
You can connect recording equipment to line out and record the signal, without having to listen to it through the device's speaker, and without the loudness of the recording changing if you change the volume control setting of the device while you are recording.
The impedance is around 100 ohms, the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at 10k ohms, and frequency response of most modern equipment is advertised as 20Hz-20kHz
This impedance level is much higher than the usual 4 or 8 ohms of a speaker, such that a speaker connected to line out essentially short circuits the op-amp.
Even if the impedances would match, yielding the theoretical maximum power transfer of 50%, the power supplied through line out is not enough to drive a speaker.
This is why you must use a amplifer speaker system with a computer line out sound card
Connection the line out to your “Audo Aux Input” on the amplifer wil not damage the ampfifer