For any given system, there are a number of ways you can do it. This generally applies to conventional subwoofers that have a plate amp. If you have a sub with an external crossover, the situation may be different.
High vs low level inputs
High level - are speaker level signals coming from an amplifier that would normally be connected to the speaker terminals
Low level - are preamp level signals
Always choose low level inputs first. There are two reasons for using high level inputs:
1. Low level inputs aren't available (typical with an integrated 2 channel amp)
2. Low level inputs don't provide enough gain
Subwoofers vary in whether they provide outputs or not. Where provided, they usually feature filtering at a pre-determined point such as 80 Hz.
Preamp > Subwoofer low level input >
> Subwoofer low level output (bass filtered out) > Power amp > Speakers
A separate power amp, or power amp inputs are required. The advantage is extra headroom both for the amp and speakers. The amp will use less power where bass is filtered out, and the drivers will use less excursion. However, this may not always give the best integration and it's better where possible to have more control of the filtering. It's preferable to have control of both the low pass on the subwoofer and the high pass on the mains. In some cases, overlap might integrate better. Also consider that the slope of the electrical filter will be added to the acoustic response of the driver. The final response will be the two combined and will often not be quite what you expect.
Normally the outputs are a filtered version of the input provided. So if you use low level inputs, you can only use low level outputs if required. The high level outputs could not be used since no signal is being fed into the high level inputs.
2 channel integrated amp
Check if you have pre outs and amplifier inputs (often marked "main in" and right next to the pre outs).
If so, use low level inputs. Two options:
1. Use a splitter on the pre outs. One set of interconnects to the sub inputs, the other back to the power amp inputs.
2. Pre outs > subwoofer inputs and low level outputs > power amp inputs
In the second option, the sub provides filtering of the signal sent to the power amps.
If you only have pre outs, then you need an external power amp to use low level inputs since your power amps inside won't get an input. Or you might have to consider high level inputs.
If you have no pre out at all, then your only choice is high level inputs.
With a receiver, it's simple. Connect the sub pre out into one of the low level inputs. You will have bass management settings on your receiver that can be used. Depending on the options available on your sub and receiver, you may choose one or the other to control the settings.
"My sub won't play loud enough when the volume is turned up."
You have a gain problem. Check the settings on your receiver. Another solution can be to use high level inputs. This can also happen to those who have used an external power amp with no dedicated preamp/crossover. In that case, an external crossover which includes variable gain should be added.
"Auto turn on does't work."
Same problem - the gain isn't high enough to activate the switch. See previous.
"Bass sounds boomy on some notes, absent on others."
There are two potential problems. One is incorrect setup with the main speakers where you might have a dip in between the sub and the mains. The other is that the room acoustics are causing peaks and dips. Most rooms have this problem and it is not easily solved.
The best solution to both problems is to set up the sub with measurements. You will need a basic measurement setup and some patience to learn how to use it. The cost is not high, you might enjoy the learning process and you will certainly enjoy the result.
More about getting a measuring setup >
1. Nearfield measurement - put the mic close to the cone of the sub and mains to isolate the sound of them from the room
This will help you set them up correctly so that you don't have dips.
2. Farfield measurement - put the mic in your listening position, then measure speakers + sub as well as showing the impact of the room
This will help you find the best placement of the sub.
This process will take some time but give you the best result.
Here is an example of the kinds of results you will see:
An example of subwoofer integration with measurements >