had a 18"sx to a 15"hd3, let me tell u fideling with the settings do different things, the 18 sounded alot better in my car ( i mean deep bass ) the 15 gets way louder, but the 18 just seemed to get those lows that i liked and it was sweet sounding, dont get me wrong i love this 15 alot and it gets low but not the noise tingling low, although outta of the subs i owned the hd3 is the best this sub is sweet
The difference you are hearing is most likely due to the enclosure not the sub. If the box for the HD3 is tuned higher than the box for the SX, it will sound louder and not as deep.
18inch = more air space, sub can move slow = lower notes sound better lol. If u had a HD3 in 18inch dont think they make that tho.... i bet it would sound better and louder..
