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If the X-540 doesn't have the power you're looking for, but you're still on a budget, consider the Logitech G-51. A step above the X-540, the Logitech-G51 is rated 155W of power. The subwoofer is a little larger than that of the X-540, and the speakers are as well.
The results of these improvements are noticeable, particularly as the volume of the system is raised. The G-51 can become muddy in dynamic musical sound tracks, but is still sufficient for most ears during any other activity. It is extremely good for games, where the larger subwoofer helps convey the thump of a large explosion.
Of course, being a little bit larger than the X-540, the G-51 is also a little bit more expensive. At about $160, the Logitech-G51 will have you digging deeper into your wallet.