|"When you get a sub, there are usually two ways of connecting the sub up to your system - line level from your preamp, or using the speaker binding posts.
1) When you use the line level inputs from your preamp, the problem is going to be the group delay (phase delay at different frequencies) of your power amplifier. No power amplifier is going to be a "perfect wire with gain". Hence, the signal phase going into the amplifier when compared to the signal phase coming out is going to show a difference that varies with frequency. This is because all components in the signal chain exhibit some form of phase shift.
If the phase delay of your power amplifier and the built-in power amplifier of the subwoofer are different at the frequency where the subwoofer kicks in, then your subwoofer and the main loudspeaker are going to have different phase response. If the difference is 180º, they will cancel at that point but if the phase difference is anywhere between zero and 180º, the result is confusion and a lack of coherence.
Hence, it is a hit and miss affair. Some subs have built-in digital parametric equalizers for amplitude correction but I've found many of them to take away some of the "aliveness" of the music. The problem is that these filters all take away something - I guess that's what a "filter" is. When there is a phase difference, there is no possible way to "turn back" phase. It can only be advanced forward in time which increases its delay. Hence, what these systems do is delay phase by up to 360º so that steady-state signals are "in step" again. Unfortunately, with music, you can't have the bassist play one beat later than the rest of the band to get back "in step".
2) The better chance of success is taking the output of the power amplifier to drive the subwoofer. It is intuitive that using the signal that drives the main loudspeaker to also drive the subwoofer will result in a greater chance of integration. Yes, but... speaker cables too exhibit inherent group delay characteristics. Really the best place to "tap off" the signal is at the end of the speaker cable, hence at the binding posts of the loudspeaker. But then what cable do you use? The audiophile will hunt through his cable closet (we all have one) and pick the "best" he has that will be "compatible" with the main speaker cable.
However, electrically the subwoofer input is not the same as a loudspeaker input. It is actually a power amplifier. Hence, a nice thick fat loudspeaker cable is actually going to sound pretty bad. It's like trying to use loudspeaker cable as interconnects between your CD player and the preamp! You will have more success using interconnects, cutting the RCAs off at both ends and using that as a jumper. However, that's still not ideal.
Interconnects are designed for high-impedance circuits. Loudspeaker cables are designed for low-impedance circuits. So the output of the main power amp driving the subwoofer amp's input is a low-impedance to high-impedance circuit. I'm sure that cable companies will eventually figure that out and sell a specialized cable for this application but in the meantime, I'm sending you a pair of loudspeaker-to-subwoofer interface cable. It's not something we generally sell but we do sell it with our subwoofers.
3) Now that you have the best chance of success with the proper electrical interface, how do you integrate the subwoofer with your speakers? The Genesis ServoSub has the best chance of success because of the inherent servo-feedback system. The servo circuit compares the incoming signal with the woofer output and the phase delay is inherently flat with respect to frequency and controlled, otherwise the servo feedback will go crazy. However, there is some phase delay due to the amplifier.
Next we need to look at the main loudspeakers and their phase behavior at the bottom of their frequency range. For all non-sealed box loudspeakers (ported, bass reflex), phase at port frequencies won't be anything like flat. Hence, your best chance for success will be to plug up the port to change the alignment to a sealed box. (I have not even begun to address the problems of integrating a ported subwoofer with a ported loudspeaker!) Some loudspeaker manufacturers supply their customers with various densities of foam port plugs to tune the bass to taste.
After plugging up the ports, figure out where the bass of the main loudspeakers starts to roll off and use that as a starting point for the G928's low-pass crossover. Because any amplified subwoofer is going to exhibit some phase delay, the only way to minimize that is going to shorten the listener distance to the subwoofer relative to the main loudspeakers. As phase delay is relative to frequency, you will have to experiment but the focus will be on the wavelength of the crossover frequency. Say you cross over at 50Hz and the group delay is 30º. The wavelength of 50Hz is 269 inches and with a phase rotation of 30º, you will need to move the subwoofer forwards by 22 inches for the woofer to "catch up" to the main loudspeakers."