Have you measured your in room frequency response? Whether or not you need a sub really depends on what your measured output is in the bass area. Your room will determine whether your drop off at 80hz, 60hz, 40hz or 20hz.
I have full range speakers, but my room kills things at 80hz, save for a giant 35hz peak. The only thing that works in my room, is to high pass the speakers to avoid the 35hz peak, and to place a sub behind the listening chair, and then EQ the sub.
Your speakers are apparently -3db at 25hz, but again, it's all about what you get in room.
You have nothing to worry about with coherence, etc, but successfully integrating a sub pretty much requires being able to take RTA measurements.
I have full range speakers, but my room kills things at 80hz, save for a giant 35hz peak. The only thing that works in my room, is to high pass the speakers to avoid the 35hz peak, and to place a sub behind the listening chair, and then EQ the sub.
Your speakers are apparently -3db at 25hz, but again, it's all about what you get in room.
You have nothing to worry about with coherence, etc, but successfully integrating a sub pretty much requires being able to take RTA measurements.