Although a speaker may publish its specs and advertise that it will do, for instance, 30 hertz, it doesn’t mean it will sound very good or as loud and rounded as you may want it. Unfortunately, almost all the speaker makers out there either outright lie or fudge the truth about what frequencies their speakers can reproduce. There isn’t a ‘technical specifications cop’ out there enforcing anything.
In general, about any speaker, the real truth is that surface area dictates how much air a speaker can move. And that pressure change is what makes bass. You can expect a 5 1/4″ speaker to go down to 120 Hz. A 6 1/2″ down to 80-100 Hz. And an 8″ to go to maybe 50-60 Hz.
The stuff that is sub bass, meaning what you really feel and not really hear, is down to 20 Hz. Below that is ultrasonic and really unusable frequencies and power robbers.
So your question can be best answered by listening to a system with and without a sub and decide whether you want one. Most folks like the addition of a sub for a nice, solid, and warm bottom end.
WHY DO I NEED A SUBWOFFER? IF I PURCHASE SPEAKERS THAT COVER THE COMPLETE AUDIO RANGE WHAT DOES THE SUBWOFFER DO?