Great question, lots of opinions. The towers I use are Dynaudios which go down to 34 Hz (-2 dB) so I cross over at 50 Hz. I use a Hsu research VTF-3R Mark 2. When I have listened to mine without my sub there is a very subtle loss of "presence" in the bottom end. Using floor standers is a bit more tricky due to (usually) a lower crossover point. I had to use a variety of calibration discs to make sure that there was a seamless crossover without humps and dips. Most monitors excel in the midrange and highs, and subs can add more presence at the bottom. There are exceptional monitors for large amounts of money that are more full range. With SVS and Hsu (and others) you can try them in your home at no risk other than the shipping. That way you can listen to them with your speakers in your room. I don't think there is another way to buy a sub since they are so dependent upon your room and your system. Good luck and happy hunting!
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