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It needs to have a good frequency response, if it cant play low enough then the sub is a waste of money (i know i have a focal cub, 8" sux...), & it has to play high enough to mesh with your other speakers. it also has to play tight clean bass, & make sure the port (if ported) doesnt create any port noise, dont want to hear air, just the clean bass. make sure the sub has enough power so u can feel the bass in your seat during explosions in movies, but make sure you dont over power it so that the buyer doesnt think turning the knob higher is better & blow it in a few days, then u have an unhappy customer looking for another company because they dont know anything about electronics. as far as looks go i dont think anyone that really likes music really cares what it looks like as long as it matches the description above, i mean come on, they're electronics not flowers.
sorry for the speach - flowerboy