Regardless of your main speaker capability in the bass, there will be instances where you will need bass re-enforcement in the room. This is going to be dependent on the room size and acoustics. The alternative is also true ( but far less often). For example, I know of a fellow a'phile who uses large Wilson Alex 2 speakers in a relatively small room. The Alex 2's have large bass drivers and can drop down into the lower 20Hz range. This fellow tried to integrate subs into his room, with little success. The reason...simple! Because the main speakers were large enough to fill his room with accurate ( this is the key point here) bass! In this particular case, it was obvious ( well to most experienced a'philes who listened to this system) that adding additional bass that essentially overloaded the room was actually undesirable! OTOH, in most other cases with speakers that can go into the low to mid 20Hz's with some ease, adding a subwoofer of commensurate quality ( Yes, that is also a very very important factor) will allow for a more defined stage and more accurate ambience retrieval.
I recently attended a demo of the new ( and superb) YG subwoofer. In the VERY large room that this was demoed, it was quite obvious that the subwoofer was a major addition to the overall SQ. The main speakers were well equipped to drop down to 25-30Hz...YG Sonja's...but add the subwoofer and the whole became a lot more than the parts. Something that is quite obvious once you experience it...and perhaps less so if you are just theorizing!
