Hooking up TWO subs

The bottom line is that playing music is a personal thing. If is sounds good to you it doesn't really matter what anybody else thinks or says.

That said, there is much more to bass than just volume level. IMO the bass wants to be time aligned with all frequencies. When a piano note is struck all frequencies propagate from a source at the same instant. If your bass speaker is farther than your main speakers, I think there will be a slight degradation in the sound. Assuming your sub is 2 feet farther from you than the main speakers, the low bass will arrive at your ears 2ms after the sound from the mains. Not only is time alignment important, so is the speed of the amp and speakers. Speed is IMO a very overlooked factor in audio. This is usually determined by the slew rate which is basically the time it takes the amp to go from 0v to maximum output. Since bass generally taxes the amp much more than highs slew rate would seem rather important to bass reproduction. So is the damping factor of an amplifier which is basically like a brake to stop the speaker when the signal has passed. The higher the damping factor the easier it is for the amp to control the speaker. Resistance in speaker wires essentially lowers the effective damping factor of an amplifier.

As an example, two amps can both put out 200 watts on a test bench, but one may be fast and one may be slow. We're only talking about a short delay here but if you give the amps quick pulses the fast one may reach full power and the slow one may not. The pulse may be over before the slow amp gets around to delivering it to it's output. If you make it a wider pulse the slow amp will have enough time to develop it's full power. In music, transients can be rather quick. When a bass drum is hit the initial attack can be very fast. If it was slow, both a fast and a slow amp might reproduce it similarly. But in real life a slow amp will not cut it. By the time the slow amps delivers the full power it will be too late for the initial attack. This will generally result in a softer sound. That said, often the bass drum is limited in production to make it softer. In that case the differences between fast and slow amps becomes less noticeable.

Remember that all of this is strictly my opinion. Obviously not everybody is in agreement here.

 

The LFE is a mono signal that comes out of a surround receiver and generally has nothing but effects on it.

As for the left and right inputs, I think in most of cases they are summed before amplification. In this way both the bass signals of the left and right preamp (or crossover) outputs can feed the sub. Of course if there are phase issues on the recording this can result in cancellation of bass.

 

(since you've volunteered for punishment )

I have found that if bass is reproduced correctly - in proportion to the highs, in-phase with the highs, up-to-speed (dsclar ) with the highs, etc. - the rest of the system is a lot easier to set up for quality reproduction, whether a $200 HTIAB or a dream system. Bass, in a small room is difficult to play properly. IMO a subwoofer, properly integrated with main speakers, makes this task easier - as I have said, it lets you embrace the room. More than one channel sound makes this task more difficult, and the benefits of integrating a subwoofer more rewarding.

Since few rooms can deliver equal bass response to any listener from five speaker locations, I always recommend that the first assumption be that a subwoofer is a great way to simplyfy the task of delivering six channels of bass. We can't assume that mixing engineers won't pan sounds containing bass anywhere in the 360 degree circle, so all speakers should at least start with equal frequency response. Five 3-way 12-inch speakers can make for a messy surround experience in most systems, trust me.

Back on topic...a second subwoofer, to obtain more headroom, is almost always a good improvement to a system.