Using a Y adapter on most, but not all, subwoofers will increase the sound level from 3 to 6 db.
Aperion Audio Subwoofer Y Cable
"Why, oh why do you need a subwoofer Y cable? Most AV receivers come with a single sub output while most powered subwoofers come equipped with left & right inputs. By using a subwoofer Y cable for both the left and right subwoofer input, you can increase the bass output of your subwoofer by about 3dB."
HDMI cable for home theater and high-performace audio video cables.
"Attach this cable to an existing subwoofer cable to split the signal into two inputs.
Instantly improves your home theater or home audio experience.
Many subwoofers have increased clarity and bass response when both inputs are fed with a mono signal."
Wye Y Cable
"Typically, powered subwoofers will come equipped with either a single RCA connector, or double RCA connectors. However, both connections typically only require a mono (single) connection. Therefore, if you have both right and left inputs, a single cable connected to the "left" input will probably work just fine. However, you can get anywhere from a 3dB to 10 dB gain from your subwoofer by plugging into both stereo inputs. To plug into both inputs with a single cable, simply purchase our Y adapter Cable along with your subwoofer cable."
This is the science behind it:
There are two formulas used to find the db ratio between sound levels.
The first is used ONLY FOR POWER (watts): 10 . log (P2/P1). This formula is used when comparing Sound Intensity, measured in watts.
The second is used ONLY FOR VOLTAGES (and SPL levels): 20 . log (V2/V1). This formula is for voltage relationships and Sound Pressure Levels.
Using a Y adapter will add two correlated signals. When you sum two equal correlated audio signals (i.e., mono sources from sub output on the receiver/preamp), you will get twice the signal level (A1 + A2 = 2A1, since A1=A2).
Therefore , since V2 = 2 V1 and the log (2) = 0.30103, it follows
20 . log (2V1/V1) = 20 . log (2) = 6dB gain in voltage.
For power, it will be
10. log (2P1/P1) = 10 . log (2) = 3dB increase in power.
These are a few advantages (some real and some claimed) to using a Y adapter:
If you leave one of the RCA inputs open, dirt may get in there.
Using a Y adapter may add a few dbs to the subwoofers's output.
Some claim that the subwoofer works better at lower levels with the Y-cable.
The auto-off mode of some older subwoofers (e.g, Velodyne CH-12) may not come on at lower levels without the use of a Y-cable.
Some older subwoofers may actually need a Y cable because the left channel L may not default to mono signal.
Another benefit of using a Y-cable at the sub input jacks may be for reducing the amount of Radio Frequency (RF) and Electomagnetic (EM) interference, because you are not leaving the other input open and active. Subwoofers can be affected by annoying buzzes and hums.
Conclusion: Since Y adapters cost a few dollars and there is no significant risk involved, I would recommend using a Y adapter (2 male, 1 female) at the subwoofer's RCA input jacks. Make sure you use a good quality Y adapter that is shielded. Otherwise, you may hear hum and noises.