We have two channel systems with dedicated left and right speakers, yet often we’re told it’s just fine to use a single subwoofer in mono. I’d like to suggest this attitude is wrong.
The single-sub advocate’s logic goes something like this. “Low frequencies are essentially non-directional so it shouldn’t matter if the subwoofer’s on the left or right side. For that matter, it shouldn’t matter if you only have one.”
That’s a reasonable argument for some situations, though not for most. I’ll share a story with you of when this works.
During one of my overseas trips, I was invited to demonstrate the (then) new BHK amplifier on a pair of expensive Magico loudspeakers. These beauties were not for the faint of heart. In fact, they cost far more than quite a number of BHK’s. I had the morning to setup these heavy metal sculptures and get them dialed in as best I could. They sounded amazing—except they hadn’t any low bass. How could they? There was no built-in active subwoofer. Try as I might, that last vestige of low-frequency energy just wasn’t going to happen without some help. Looking around the dealer’s well-stocked listening room I noticed a massive JL sub sitting off to one side. It didn’t take long to connect it to the BHK preamp’s unused outputs and dial it in. By keeping the sub’s top frequency low—perhaps 35Hz—the Magicos “magically” had bass. It was impossible to tell the low notes were coming from one side of the room, which supports the idea of a single sub.
Listeners to my talk and demonstration, including reviewers, congratulated me on the demonstration. They’d never heard the Magicos sound that good: that full, that great. Of course, the BHK system did its magic as it expected, but that extra touch of bass sealed the deal.
All that said, I still advocate stereo subwoofers for most systems. Generally, when we add a subwoofer, we’re running the top of the sub a bit higher than I did. And, any higher, and the very top of the sub becomes directional and your soundstage a bit less believable with only one sub.
If you want the rest of the story, watch this video Do you need two subwoofers?