Sennheiser HD-540 Reference II
Not much is documented about the Sennheiser HD-540 Reference II. The best record is a single thorough review in Audio (June 1992, p.106), which also printed a full spec list.
Audio put the HD-540 Reference II in Sennheiser's "open-air" tradition. It is a dynamic, circumaural headphone with no tight seal at the rear or front of the diaphragm, and slots in the outer housing let the rear of the transducer breathe. The specs: 290 ohms DC resistance, positive absolute polarity, and a 9-foot straight cord running from both earcups (stereo mini-plug with 1/4-inch adaptor). The headband slides in a detented bail. Weight was 6 oz., and the 1992 price was $199. The earcups were gray with brushed chrome, and the cushions were velvet.
The reviewer called the Sennheisers an excellent choice for long-term listening and an excellent value at the price (Audio, June 1992, p.108). Measurements found a dip near 2.8 kHz that slightly obscured an otherwise good upper-range response, and the impulse output confirmed positive polarity. Beyond this review, surviving information on the HD-540 Reference II is thin. No production dates or further context appear to be recorded.
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