A single sub does not necessarily need to be located exactly centred between the main speakers. This is only required when the frequency and slope of the sub's low pass filter are too high and too shallow respectively. The sub then becomes locatable by ear. This is supported by the fact that many people take advantage of room acoustics by locating their sub(s) in corners, with no negative effects.
There are many systems in which the addition of a subwoofer seems to detract from the reality of the performance. IMHO this is almost always due to two things ... the sub is not good enough and/or the sub's integration has not been optimised. The sub must be of high quality, well damped, mounted in a suitably large and rigid box, and be driven by an amplifier with plenty of headroom. The sub must have a Q of between 0.5 and 0.7 (not the typical 0.7+ used in many home theatre subs). For music it does not need to go down to 0Hz but will ideally be flat down to 30Hz. IME sealed subs are preferable for music, regardless of whether the mains speakers are sealed or ported.
Optimising the integration of the sub includes: location; feet/platform; LPF frequency; LPF slope; phase; gain; and optionally HPF frequency and slope on the main speakers. The optimal settings for all of these will largely depend on your main speakers and your room acoustics and will be different from system to system and room to room. This is why so few people ever get to hear their subs truly optimised. It is very time consuming, confusing and even frustrating to do by ear alone. I use the Acoustisoft ETF5 measurement software and find it saves me a great deal of time and gets me to optimisation levels I probably couldn't attain by ear alone. Unfortunately, most sub owners seem to have the sub's gain and/or LPF frequency set a little too high and this causes muddied lower mids and a bloated bass, with a discontinuity between sub and mains speakers. An excellent sub, optimally integrated with excellent main speakers (either stand mounted or floor standers) adds a dimensionality, scale, effortlessness and openness that can't be matched by even $$$$ 3-way floorstanders on their own (except perhaps very high end speakers with high quality 10"+ woofers and tri-amping). The music just comes alive! Often the mids will also be improved with better body and tonality and the overall balance is more natural and lifelike.
My single sub (DirectServo, 12" driver, rigid & sealed 2 cu.ft. box, 24db/oct LPF, switchable Q 0.5/0.7/0.9) is located just inside my right speaker. My speakers are very high quality standmounts (6.5" SS Revelator, rigid & sealed 0.75 cu.ft. box). I let the main speakers run full range and have the sub's LPF set to 63Hz. In my large-ish room this provides a +/-5db response from 16Hz on up. This has provided a seamless integration and, if you were to walk in blindfolded, you would swear you were listening to very large floorstanders rather than a sub with standmounts. This setup offers the excellent imaging that high quality standmounts are known for, along with the great bass that only the best floorstanders are capable of.
I am no bass-head, but one thing I have found is that once you have finished optimising the sub/mains integration and it measures flat, then turn up the sub's gain just a little (half of one o'clock position or 15 degrees in my case). Our ear/brain seems to find this more realistic and lifelike than a perfectly flat response.
Sorry for raving on, I hope this is useful to someone.
