Several days ago I promised I'd tell you how I connected the mystery subwoofer to the system and why.
The subwoofer is a sealed box powered subwoofer I am auditioning from a local company and it is really an excellent sounding product, easy to seamlessly integrate into the system and as musical as I have ever heard from such a device. The one downside to its design is the lack of a high-level input. Supplied is only the standard left and right inputs as well as an LFE input, all through cheezy RCA connectors. Believe me, I've tongue lashed the appropriate people.
Having said that I still want to connect the sub up through high level inputs for a couple of reasons: I like the sonic qualities of this connection scheme and it allows us to use a single analog output to feed the power amplifier - rather than two outputs, one for the main amp and the other for the subwoofer.
So what is a high level input? Basically you take the output of your power amplifier and feed it into the RCA connectors on the back of the subwoofer. You're not using any of your main power amplifier's wattage to do this, only the signal. But the first problem you encounter is the signal coming out of a power amplifier is quite a bit louder than what is going into the power amplifier - about 30 times louder to be specific. So if your sub has high level inputs - which will look like speaker binding posts or, if you have a REL sub, an extra cable they supply - all that's inside is a simple resistor divider network that reduces the loud output of your power amplifier down to the same level as its input. In other words, inside is a divider network of 30, consisting of nothing more than a couple of resistors (typically a 30K and a 1K). This network will also have a couple of blocking capacitors in case the amplifier you're using is a balanced output.
One of the main advantages of using this connection method is better blending between the main loudspeaker and the sub. Because you're using the output of your power amplifier to feed the sub, whatever sonic qualities your sub imparts on the music will be transferred to your subwoofer. Hopefully your power amp isn't imparting too much of its personality into the music, but they all do some and this carries forward those sonic attributes or colorations to help blend between the two speakers.
It's instructive to remember that adding a subwoofer to your two-channel music system is really extending the driver compliment in your main speakers. If you have a 2-way loudspeaker (woofer and tweeter), you'll now have a 3-way and so on. If you keep this fact in mind, it'll make more sense to you that whatever you do you'll want to get as close to matching your main speaker as possible so it sounds like one enclosure.
Lastly, if you can reduce the number of interconnects between your preamp or DAC that is feeding the main outs, you're usually better off. The more cables you connect to your preamp or DAC, the greater the risk of degradation you experience - and using the subwoofer's high-level inputs eliminates the need for an extra connecting cable and keeps the speaker/main amp system as integrated as possible.