I'd like my mains to play an octave below the crossover point if possible. Crossovers are not brick walls that drop to zero instantaneously on either side; they roll off gradually, so there can be significant energy an octave above and below the crossover frequency. It is easier to integrate subs when there is goodly overlap with the range that the sub can handle.
Copy of my old standing response to "why subs?":
I use subs, and have for decades, for all the usual reasons:
- Very (perhaps extremely) few "large" speakers actually play well below 40 Hz let alone 20 Hz. They distort heavily when presented with large bass signals (which most are -- see Fletcher-Munson) and driving them hard down low robs headroom for higher frequencies and causes distortion well above the fundamental signal frequency (harmonic and nasty intermodulation). Subs typically enable the mains to operate with much lower distortion.
- Very rare is the room setup such that the best place for stereo imaging and soundfield is the best place for the subs (or deep bass drivers) to counter room modes and such. Having independent subs provides placement options to smooth the in-room response. It is almost impossible to counter a null without subs (typically must move the MLP or change the room's dimensions though there are purpose-built panels that can also work). This is one of the things that led me to subs despite having quite capable mains.
- Powered subs offload the main amplifiers of the need to provide deep bass energy, providing more headroom and cleaner sound from the amplifiers.
- Music (let alone action movies) often contains deep bass content even if it is not real obvious. Kick drums, tympani, organ, sure, but also piano hammer strikes, plucked strings, beat patterns from instruments playing together, etc. May not really notice when they are there but usually obvious when they are taken away. Having subs fill in the bottom octave or three can make a difference.
- Purpose-built subs can provide high output cleanly at relatively low cost. The amplifiers and drivers need only cover a fairly limited frequency range so have fewer constraints upon them than woofers in a full-range system.
I do prefer main speakers with fairly deep bass and always have. Crossovers are not brick walls so a fair amount of energy still comes from the mains an octave below the crossover frequency. Higher-order crossovers allow you to reduce the overlap, but I still like having the capability. I have never really understood the idea of running "passive" bi-amping as implemented by an AVR (sending full-range signals to multiple channels and letting the speaker's crossovers separate frequency bands -- wastes amplifier headroom and seems to me of little benefit). Nor do I agree with the "plus" setting putting subs and mains in parallel; again, my idea has always been to isolate the two for the reasons above.
My first sub was a DIY design using an Infinity IRS woofer with my own control box to provide the crossover and a servo circuit using the second voice coil of the woofer. I had a Hafler DH-220 around so also incorporated a circuit to bridge it for use as a subwoofer amp. It worked well and the -3 dB point was ~16 Hz. I now run four small (F12) Rythmik subs using a similar (but updated) servo design with my Revel Salon2's and am happy with the result.
FWIWFM/IME/IMO/my 0.000001 cent (microcent) - Don