So the answer is... It will be almost 2x (3db) louder with 2 subs on the same amp as opposed to 1 on the same amp?
It won't *sound* two times louder...it takes 6db to 10db to be
perceived as "twice as loud" by the human ear.
And, as I and lilmaniac pointed out.....those are only
theoretical numbers. Real world results will depends on many, many, many different variables.
And even then...."on the same amp" is a little limiting, since most amps will not produce the same power output into multiple loads (PG Xenon and JL Audio Slash amps will, however). Say you, for example, have a single DVC 4ohm/coil sub wired in parallel to 2ohm, and an amp that's only stable down to 2ohms. Well, if that amp puts out 500w @ 2ohm and 250w @ 4ohm, then that single DVC 4ohm speaker wired to 2ohm will be recieving 500w. Now, add a second DVC 4ohm/coil speaker and you're only option is to wire the subwoofers to 4ohm (while still presenting a "safe" load to the amplifier), where the pair will only receive 250w....or 125w each.
It is correctly stated as two subs off of the same
power will yield a theoretical 3db increase in output. Not off the same amp, but the same
power.