Actually, you don't realize any more gain or power from your sub by using a "Y" connector. While you may be able to turn the gain control down somewhat, the amp in the sub still only puts out whatever it can, regardless of the level setting.
Mark is correct.
Just to clarify here, what you are actually getting on most subs by using a y-cable to feed the L/R inputs is input signal gain since each input is being summed before going to the power amp. You will actually get up to 6dB of voltage gain depending on how the summing is done (assuming no limiters or active gain control circuits are present). This can yield a 3dB increase in output level, all things being equal. However, this also brings you 6dB closer to clipping the sub preamp and 3dB closer to reaching the dynamic output limits of the subwoofer. I usually recommend using a ysplitter in the case where your preamp sub out has weak signal levels (really not the case anymore). In reality, your only upping the input level, while keeping all other things constant (IE. volume controls).
As a side note, when I used to design audio interfaces, I usually added an attenuator after the mixing stage to eliminate the added gain. I am not sure why they don't do that with sub amps?
As another side note of interest:
When you sum two correlated audio sources: (IE. mono sources):
You get: A1 + A2 (where A1=A2) = 2A1 or twice the signal level
thus you get 20*log(2) = 6dB gain in voltage; for power it would be 10log(2) or 3dB
When you sum two Non Correlated audio sources (IE. stereo sources)
You get: sqrt(A1^2 + A2^2)
For example: Let A1 =1 and A2 = 1
The equivalent output for summing two non correlated audio sources is:
sqrt(1^2+1^2) = 1.41 times the original signal or
20*log(1.41) = 3dB voltage gain or 1.5dB of power gain based on 10*log....
Hope this helps and it appears I need to update our setup articles on this topic for greater detail.
