Ok, there are a couple of ways that it can be done, for probably a lot less money than you think:
1) Subwoofers with high-pass outputs:
It really can’t get much easier than this. Essentially, you are inserting a subwoofer cabinet between your amplifier and speakers, using the existing speaker cables.
You connect speaker cables from your amplifier outputs to the subwoofer. Then another set of speaker cables from the subwoofer to your speakers. Job done!
Please note that this cannot be done with any subwoofer. It must be specifically designed to be able to be used in this manner.
Usually, the subwoofer will consist of a single ‘dual-coil’ driver which allows for a stereo signal to be sent to the sub. The sub will then have a pair (left and right) of ‘high-pass’ outputs which you connect to your speakers.
The high-pass filter network ensures that bass frequencies are not passed through to your speakers (kind of like a x-over which has been pre-set for you).
The big advantage of this type of setup is that it is inexpensive.
Assuming you already have a suitable power amplifier and existing speakers, you are essentially just adding a subwoofer (and possibly some cable) to the existing setup. It is also extremely easy to perform this type of upgrade.
The disadvantage of this setup is that you have no control over the sub.
You are not able to adjust the subwoofer volume independently of the speakers. Your x-over point is also not adjustable, as it is determined by the high-pass frequency set by the subwoofer cabinet.
2) Use a 3-channel amplifier:
While 3-channel amplifiers are not overly common, they can be very useful in circumstances like this.
A 3-channel amplifier will essentially allow you to feed it a stereo audio signal and in return, it will dish out separate power for left, right and sub.
Depending on the amplifier, you may be able to adjust the x-over frequency for the subwoofer or it may be fixed at a certain frequency.
The advantages of using a 3-channel amp are that you are getting your amplifier and x-over in a single device, so it’s one less item that you need to install, power, connect and configure in order to get the system going.
You also have separate level control for your left, right and sub outputs, so you can alter the volume of your sub independently of your speakers.
There are no real disadvantages to using a 3-channel amplifier in this way. Although it may be a bit more expensive than using a subwoofer with high-pass outputs as outlined in option one, you will have greater number of subwoofer cabinets to chose from as you can use any standard passive sub, without requiring dual-coil drivers or in-built high-pass outputs.
You will have less functionality than if you were to go for a separate x-over, however for budget installations, the additional functionality may not be required.
Audio Logistics offer solutions for both of the above configurations:
For solution #1, a Void Acoustics VENU SUB 12″ dual-coil subwoofer will work in a number of environments which require low to mid-level bass reproduction.
Link: Void Acoustics VENU
For solution #2, The Audac DPA73 & DPA153 3-channel power amplifiers are great for background music applications. If more power is required, the Audac SMQ range of 4-channel amplifiers can be set to run in 3-channel mode. As well as offering additional power output, the inbuilt DSP also provides additional functionality including; EQ, x-overs, delay & limiting.
Link: Audac DPA
Link: Audac SMQ