But just buying better speakers that don't need a sub in the first place would probably be a better plan
Once you tried a good sub maybe you change your mind as me ... any bookshelf speaker needs a sub
Buy your bookshelf speaker thinking at good high and mid frequencies response, dispersion, etc ... usually the design compromises can't do it all very well. So, let the bass to a sub.
My opinon after years of "urban legends" about subs ... and how i change my mind with real experiences.