A cap to block DC is usually only at the amp output. I've never seen one on the speaker for that purpose. And the capacitor has to be large enough to pass the lowest frequency of interest without significant attenuation into the low impedance of the speaker. This requires a large capacitor for the lower frequencies.They're usually for DC filtering on the speaker input. If you put a small capacitor in series with the speaker, it eliminates any DC in the input whilst preserving the audio signal.
Why are capacitors at times connected to the terminals of speakers and how do they vary in their different sizes