If you run a sub too high in frequency, you can tell it's a separate sound source, especially if it's far away. But closer subs can be crossed-over higher. In fact, I like having a sub about 5 feet away from each main, with its low-pass set around 90Hz to 120Hz. There's no way to tell what's sub and what's mains - they sound like one common source - but the midbass is made noticeably smoother. Subs further away must be low-passed at a lower frequency, and are generally more effective at smoothing those lower frequencies as well.I've never liked using a single sub. It almost never can be setup right. A pair is a lot better, even if just placed a few feet from the mains. Better still to add a couple more, further away and crossed lower. The bottom line is, the results you'll get from subs is largely a function of how many you're using, where they're crossed and where and where they're located.