If you have a 2 channel system you may consider adding a subwoofer at some point. If you do, you should also consider adding a separate 2 channel crossover.
A 2 channel system will benefit from the addition of a quality subwoofer. But to maximize the benefits the main speakers should also be highpassed. That is, relieved of the deepest bass responsibilities. Some two channel preamps do offer bass management capabilities that will allow the user to select various crossover points for the speakers----just like a AVR(audio video receiver). In this case a separate crossover is not needed. However, the majority of 2 channel preamps do not have this feature. The addition of a quality crossover will relieve the 2 channel amplifier and the main speakers of the deepest bass. This, in turn, will tend to 'clean up" the mid and upper frequency performance of the main speakers---particularly at higher volume levels.
You can find 2 channel crossovers in the "pro audio" world under $100. Or, "audiophile" grade crossovers from a company like http://www.marchandelec.com/ Match the quality of the crossover to the quality of the system it will be integrated in to and audible benefits are sure to follow.
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