That said most subs these days are built pretty well and perform evenly. Almost any stand alone sub will beat the daylights out of one that comes in a HTIB home theater in a box sub. So take a look at this assortment and pick one out that fits your budget and room size.
Of the ones shown our favorites would be the following
A good entry level sub from Polk audio. Great sound and a fantastic price with free shipping if you have Amazon prime
Polk Audio PSW10 10-Inch Monitor Series Powered Subwoofer (Single, Black)
Energy is one of our favorites. When we were at The Speaker Company our subs were modeled after these.
Energy ESW-M8 NA 1,200-Watt Subwoofer