The subwoofer is the only speaker in a home theater that can be bought from a different brand. No worries about the maintenance of the tone in relation to other boxes in the system. The band of low frequencies it reproduces is outside the region where the differences in tone can be noticed.
The power of the subwoofer should be twice the power per channel of the receiver. Example: if the receiver has 100W of power per channel, the subwoofer should be 200W. Although not a rule (there are exceptions), the larger the woofer used, the greater the possibility of the sub to achieve lower frequency waves and disperse to a greater area.
The chosen sub can be either of acoustic suspension type (enclosed) or bass-reflex (with air outlet duct). But only you can decide if whether you want a model with more musical performance with faster beats and dry or impactful and full-bodied and deep bass for movies. There are good options for both types.
In doubt, visit a showroom to shop, listen and compare the performance of subwoofers available on the market for a good choice. You may get more information at our Receivers and Speakers shopping guide.