You don’t necessarily - and probably won’t - get any significant extension by adding the DB 15 to your SL 112.
To understand why, let’s look at the frequency response specs, as supplied by
Sweetwater’s website. Yes, I know there’s only so much stock you can put in factory numbers, but let’s go with it for the sake of this discussion.
The SL 112’s extension goes down to 37 Hz, while the DB 115 gets down to 32. So, you’ll have two cabinets operating between 3 kHz (the 115’s upper limit) and 37 Hz, with only one (the DB-115) operating between 37-33 Hz. Thus,
the range between 37-33 Hz will quite naturally be audibly attenuated, since only one speaker is covering it.
Even if this doesn’t quite play out in “real life” as predicted on paper, the difference between extension to 37 Hz vs. 33 Hz is only about 1/6 octave. That’s the equivalent of one step in the music scale - i.e., from an E to a D, which is pretty insignificant.
Thus, if you want better extension, the answer is what @rodsnhawgs indicated: A cabinet designed and equipped to do it. But that often will require a larger, heavier cabinet - and possibly a more powerful amplifier to go with it. Or, a properly set up bi-amplified system with a dedicated subwoofer.
Alternately, if all you’re concerned with is the SL 112’s “mid-heavy” response, adding a parametric EQ pedal could tame that, making it sound better to your ears. However, that may end up with you needing to drive the speaker harder to make for lost SPL. In that case, a second SL 112 as @agedhorse recommended would be the ticket.
Regards,
Wayne A. Pflughaupt
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