Studio-grade sound for a lot less than new. Which used headphones to buy, and what each pair needs to sound right.

Some will and some won't, and it comes down to impedance and sensitivity. Low-impedance or very sensitive pairs, like the 32-ohm Grado SR60 or the 60-ohm Koss Porta Pro, play well from a phone. Studio classics at 300 to 600 ohms, like the AKG K 240 Monitor or the 1980s Beyerdynamic DT 880, need a headphone amp and sound thin and quiet without one. The original 2,000-ohm Sennheiser HD 414 and the AKG K 1000 were sold to run from an amp's speaker outputs, and electrostatics need their own energizer. Every model page here says which group a pair is in.

Not on paper. Koss's written terms cover only the initial user or purchaser, there's a handling fee, and Koss may replace a discontinued model with a current one instead of repairing it. Some owners report Koss being lenient, but treat a secondhand Koss as uncovered and judge it on its condition.

Count the pins on the plug. Normal bias, at about 230 volts, uses a 6-pin plug, and Pro bias, at about 580 volts, uses a 5-pin plug. On a Lambda, also check the nameplate on top of the cup under the headband arc, which is blank on a Normal-bias SR-Lambda. Cables can be swapped, so if the plug and the nameplate disagree, ask the seller. A Pro headphone will play in a Normal socket, more quietly, but a Normal one shouldn't go into a Pro socket through an adapter.

Often, yes. Sennheiser still sells pads for the HD 414 and HD 424, Beyerdynamic sells pads, cables and even drivers for the DT 100, DT 880 and DT 990, Koss sells cushions for the Pro4AA, Porta Pro and ESP/950, and Grado sells every cushion it makes. Others have nothing. Beyerdynamic lists no DT 48 or DT 220 parts, and Stax Japan has ended repairs and parts for its classic models, so on those, judge the pads in the photos closely.

Usually it's the cable or a connector, not the driver. On Sennheiser's 1980s and 1990s open headphones, the two-pin cup connectors lose contact as their springs relax, and bending them back fixes it. Grados tend to break their wired-in cable just above the plug or at the Y-split. On a Normal-bias Stax, one side can play louder for a while after switch-on, which settles down as the diaphragms charge.

For current bestsellers, yes. Sennheiser, Sony and Audio-Technica all warn about counterfeits, and Sennheiser says the serial for its HD 600 and HD 650 is on the box only. We found no documented fakes of the vintage models on this site. The closest thing is the Alessandro MS1 and MS2, genuine Grado-made headphones that look like the SR series but lack Grado's model button.