"Home / HI-FI / 2-channel / Audiophile / Surround Sound systems: There is ONLY ONE truly correct way to "add a sub" to a system in an controlled listening room situation: you must correctly cross over the 2 sealed cabinets; and their timing must be correct. ANY other method will lessen the focus and clarity you have tried so hard to preserve.
with home theater setups where the customer THINKS that the front speakers are "full range". Even so, the BEST approach is to seal the ports, operate the 5 channels as "small", crossover at 80 (or even a bit higher, but NEVER lower)
Actually adding a crossover is really a WIN-WIN situation:
WIN # 1) Since you are now NOT putting in 20 Hz - 80 Hz into the mains you are not using up the available LF cone movement with bass, so the LF cone in your mains is able to play its higher freqs (up to IT'S crossover point) much more cleanly. You get an apparent 6dB or more dynamic range. You can play your system LOUDER, and also with less compression distortion in the LF driver when you're having that Saturday night dance party and you're playing urban bass technopop at 110 dB. Really.
WIN # 2) Since you are not putting bass into that same driver you are not Doppler modulating everything between 80 and 600, or whatever the next crossover point is. This means cleaner mids. By far.
WIN #3) You are not sucking current out of your main power amp at low frequencies, so there is more current reserve to play those highs louder...
WIN # 4) Since the cones aren't moving as far at the low freqs the driver itself is not generating as much back EMF therefore the damping factor and all of its issues are greatly negated. And you don't need to run silver plated cold water pipes to your mains as speaker wires because there is less current draw by the speakers.
WIN # 5) Freqs below 80 are now NOT causing transient intermodulation distortion with the higher freqs (and vice versa) in your power amp. Cleaner still."