connecting a subwoofer: which output, and which settings matter?
A subwoofer joins the system by one of two documented routes. Then crossover, level, phase and placement decide whether you hear it, or just the music.
A subwoofer is the one box in a hi-fi system that can make everything worse. Get it right and the whole system sounds bigger and oddly more relaxed, and you stop thinking about bass at all. Get it wrong and every record announces that there is a box in the corner, thumping a beat behind the music on exactly the tracks where you wanted it to vanish. The difference is almost never the subwoofer itself. It is the socket you plugged it into and the four controls on the back.
There are two jobs here and they want doing in order. First, get a signal into the subwoofer by a route both manufacturers document, which for most systems means one RCA cable from a socket marked SUB OUT. Then set crossover, level and phase, and move the cabinet around until the bass stops sounding like a separate instrument playing along. The first job takes five minutes and has a right answer. The second takes an evening and doesn't, which is why most of what follows is about the second.
start by working out which socket you actually have
Check the subwoofer is active first. An active one has a mains lead and a panel carrying level and crossover controls, with its own amplifier inside; a passive one has none of that and cannot be driven from a line-level SUB OUT however neatly the plugs fit. Nearly every domestic subwoofer sold today is active, so this is a ten-second look rather than a worry.
What you want is a socket marked SUB OUT, SUBWOOFER or LFE OUT on the amplifier, preamplifier, receiver or streamer, and a single screened RCA subwoofer cable from there to the input its maker nominates. What matters about that output is that it follows the volume control, so the subwoofer rises and falls with the speakers. A fixed output, the kind marked TAPE OUT or REC OUT, sits at one level whatever the volume does, leaving the subwoofer at a constant loudness while everything else moves around it. The plugs are identical and nothing warns you.
Then there is the trap that catches owners of a subwoofer with two line inputs marked L and R. You do not need a Y-adapter, and fitting one because an empty socket is nagging at you will not help. One of the two takes a mono feed on its own, and which one depends entirely on who built it. Wharfedale mark the pair 'L and LFE/R' on the SW-12, so the right-hand socket is the one. SVS do the same on the SB-1000 Pro, L and R with the R as LFE. Emotiva go the other way on the Airmotiv RS13, where it's the left that doubles as LFE. Q Acoustics sidestep the argument on the QB12 by fitting one line input and calling it LFE. Read the socket, not a diagram you found somewhere.
high level is a technique, not a spare socket
High level, or speaker level, takes the subwoofer's signal from the amplifier's speaker terminals rather than a line output. Its input impedance is very high, so it draws almost no power and the main speakers carry on exactly as before. The case for it is that the subwoofer then hears what the speakers hear, through the same amplifier, and REL build their whole range around that idea: the T/5 specification sheet lists Hi Level on a Neutrik Speakon alongside the phono inputs, and REL's own order of preference puts the Speakon first.
It is not somewhere to go because the sub output is already occupied, though. Where an amplifier has a line-level subwoofer output that is the better connection, and our planner will say so. High level earns its place when there is no sub output at all, or when the subwoofer's maker prefers it.
The wiring is where real care is needed, and this is the one place in this guide where the manual genuinely is the final word. High-level inputs share a common ground between the channels, so a fully balanced amplifier, and equally a bridged, differential or monoblock design, can present speaker negatives that are not ground at all. Wire a subwoofer across those as though it were a conventional stereo amplifier and you can damage something. Never use a chassis screw, and never touch the protective mains earth, which protects people and does not cure hum. The connector varies too: REL and Sonus faber fit a Neutrik Speakon and supply the lead, while SVS and Emotiva label theirs simply as high level inputs. If you own a REL, our dedicated REL guide works through their scheme properly.
crossover and level are really one decision
The low-pass control decides how high the subwoofer keeps playing, and the place to start is your main speakers' own quoted bass extension rather than a figure from somebody else's system. Set it too high and the two overlap, which sounds thick and, worse, makes the subwoofer easy to point at by ear, which is the one thing you were trying to prevent. Set it too low and you leave a hole between where the speakers give up and where the subwoofer starts, which people then try to fill by raising the level, putting both problems in the room at once.
Level wants to start lower than feels right. Bring it up until low notes gain weight, then play four or five different records rather than the one bass-heavy track that made you want a subwoofer, because a setting that sounds magnificent on that will be absurd across an album. You are aiming for a subwoofer you notice when it is switched off.
phase and placement do more than any cable
Phase lines the subwoofer up with the speakers through the band where they overlap. A switch gives 0 or 180 degrees and a dial gives everything between, and the way to choose is to sit where you actually listen and take whichever hands over most smoothly rather than whichever makes the most bass. Those are different settings more often than you would expect.
Placement is the lever with the longest travel, and it costs nothing. Bass this low is shaped by the room far more than by the equipment: boundaries reinforce output, and room modes leave a peak in one seat and near-cancellation in the next, so moving the cabinet a foot does more than any cable will. A corner gives the most output and the least control, exaggerating whatever peak the room already had, while somewhere near the main speakers tends to make integration easier. The old trick of standing the subwoofer on the listening seat and crawling the room to find where it sounds best really does work, as a starting point rather than a measurement, because no room is truly symmetrical.
when it plays nothing, hums or booms
Silence is usually not a fault. Check mains, input selection, the cable at both ends, the level control, and whether the source has its subwoofer output enabled at all, because plenty need switching on in a menu. Auto-standby is the other classic, since a subwoofer that wakes on signal may need more level before it notices and will sit there looking broken until it gets it. Some go further: the Emotiva RS13 works out which input is in use at power-on and mutes the other, so a subwoofer that was fine yesterday can be silent today because you changed socket.
Bloat and one-note thump mean too much level, too high a crossover or too close a wall, usually all three. Take the level down first, because it costs nothing and tells you immediately. Hum that arrives with the cable is a ground loop, and the cure is in routing and in which paths share an earth, never in lifting the mains earth. Power down between changes, try a cable you trust, and pull the other signal paths out one at a time until the noise goes.
the two-minute checklist
Confirm the subwoofer is active, then find a sub output that follows the volume and avoid any fixed record output. With two line inputs, use the single socket its maker nominates rather than a Y-adapter: Wharfedale and SVS put it on the right, Emotiva on the left. Never wire high level across a balanced, bridged or monoblock amplifier without the manual open. Start the level low, take the crossover from your speakers' real bass extension, set phase by the smoothest handover, and move the cabinet before you change anything else.
So, which output and which settings matter? The sub output nearly always, and after that level and placement far more than the rest put together. Our planner finds the documented route between your own two boxes and names the cable it needs, and adding a QB12 to a pair of active speakers shows it settled on real equipment. The reward is a strange one: you stop hearing the subwoofer and start hearing everything else. Happy listening.