It just depends it you want lower bass than the speakers are capable of producing. The subwoofer compliments the speakers, it doesn't make the speakers sound any better.
Many vintage speakers from the 70s had large 10+ woofers for lower bass. Now smaller speakers that are easier to place plus a powered subwoofer with it's own amplifier for better bass management are a popular option.
Try connecting a powered subwoofer, if you don't like it you can often return it.
Great deal on the very good 10" JBL SUB 550 $599 $199.
Or a decent 10" Klipsch R-100SW $175.
Or a much more basic Jamo S 810 SUB $109 slim subwoofer or even slimmer 8" Monoprice $129 can be placed under or behind furniture.