Should I get a second sub or a higher quality single sub?

As a general principle, the in-room smoothness in the bass region improves as the number of independent bass sources increases.  So two subs intelligently placed (far apart instead of close together, for instance) will have about half as much variation in in-room frequency response as one sub.  And four subs will have about half as much in-room variation as two subs. 

One of my four-sub customers recently had a technician come over to calibrate his powerful room correction system.  He started with all controls flat.  The ONLY correction the technician found to be needed was a simple level adjustment - the in-room response of the four subs was inherently smooth enough that no further EQ was needed!  The technician said that he'd never seen anything like it.  And this smoothness holds up well throughout the room, unlike EQing a single sub, which usually makes the response worse at other locations within the room. 

The relevant point being, two subs intelligently placed and properly adjusted will be much smoother in-room than a single larger sub.  And smooth bass = fast bass.   (Because low frequency systems in rooms are essentially minimum-phase, when we fix the frequency response we have also fixed the time domain response, according to physicist Earl Geddes, from whom I learned about the advantages of a distributed multi-sub system).

Duke

dealer/manufacturer