I'm gonna assume you mean the difference between an spl sub and a sq sub...most sq subs sound better on music and have a lower fs(free air resonance), they require less power(usually)..some examples of sq subs would be an fi q, re mx, re xxx...alot of it comes down to install and tuning..I run massive dmx's witch are generally considered spl subs, but I have them in a ported box that's tuned low and they actually sound pretty good on music. They are what I would call a sql sub. A true spl sub ie: rd audio superheavy weight, re mt, handle massive amounts of power and normally require higher tuned enclosures( 60 plus hz ). They generally sound like shit on music(because of the higher tuning), and can take gobbs of power in short bursts(for burbing). This is just my opinion, like I said alot of it(90%) comes down to install and tuning.