The subwoofer can go under the couch (or wherever)
Not if you're looking for ideal performance
If it fits (or if your sub is a couch), I suppose it can, but the notion that subwoofer sound is always impossible to localize is just wrong. Even a true sub with its crossover set to 80 Hz or even lower may occasionally emit audible, potentially localizable sound an octave (or more) higher, which inevitably draws the ear off-screen. Ideally, subs should be placed on the same plane or behind the front left-center-right front speaker array, and no more than perhaps 60º from the center line.
