I think good subs that can be tweaked in to mate with your room & system do make sense and can add to the enjoyment in a meaningful way. Less adjustable subs or trying to do to much from a single sub can be more of a distraction than a benefit. And it depends on your taste in music. If there is little to be heard then a sub adds little (duhh, ok so that ain't such an epiphany).
To answer your question directly, if I had to choose between better sound overall or adding a sub, I'd take the former, but if I could afford the additional extravagence of one or two good subs I'd go for it. IMHO, YMMV, DEYS.