I put 4 of these in a sealed quad box and it hit good. I had 2 MTX T9515-04 in a custom vented box tuned at 38.38 hz and the MTX subs had that insane bass and beat the sealed on low end shaking. For the recommended sealed enclosure JL specs, the sealed box sounds good when it hits 30 Hz, but the bass sounds funny when higher notes are hit, it makes you worry if the voice coil will be damaged. I'd never buy JL Audio subs again. Their components and HD amps are phenomenal though.