Used subwoofer buyer's guide: drivers, cabinets and real power handling
Subwoofers are the hardest working boxes in any rig. These are the checks that separate a bargain from a pair of expensive cabinets with tired coils.
Why subwoofers age differently
A line array element spends most of its life at moderate level. A subwoofer spends its life at the limit. Surrounds soften, spiders sag, coils darken and the gap collects dust. None of that is visible in a photo, and all of it changes the sound.
The sweep test
A slow sine sweep from 25 Hz to 120 Hz at moderate level reveals more than any visual inspection. Listen for a scrape at a specific frequency — that is a coil rubbing. Listen for a rattle that appears only at high excursion — that is often a loose spider or a cabinet joint. A sub that sounds clean through the sweep and produces no rattle at full power is a usable sub.
Measure impedance
Compare each driver's DC resistance to the model specification and to the other cabinets in the lot. A driver that reads significantly low has a shorted turn; one that reads high has a damaged coil or corroded terminals. Matching pairs matter for cardioid arrays and for predictable power sharing.
Cabinet condition
Check the corners, the baffle and the port. Water damage shows as swelling at the base and rust on the grille staples. A cabinet that has been stored outside will often smell of damp when the grille is removed. Push on the baffle — a soft spot means the cabinet has taken an impact.
Real power handling
Manufacturers quote peak numbers that assume a perfect driver in a perfect box. In used equipment, assume 70% of the rated AES power as a safe continuous figure. If the amplifier is sized to the peak rating, the sub will be working harder than it should, especially with compressed modern music.
What to ask the seller
Ask for the serial number, the driver part numbers, a sweep video and the amplifier preset used. Ask whether the cabinets have been reconed and with which kit. A recone is not automatically bad — an incorrect recone is.
Pairing and resale
Matching pairs hold value better than singles and are easier to deploy. If you are buying four, buy them as two matched pairs from the same fleet rather than four singles from four sellers.