Classic designed Bose 901 speakers are still in production 40 years later.There are detail differences among the various versions, chiefly the cabinet and grille styling and the internal structure of the cabinet. Most have a wood or wood-looking cabinet; a few were built having a glossy piano black finish.
901 series is a different animal in that sound quality was a real, driving force in the design and development of the speaker system. Since the current production is the Series VI; and it has been since '88. Bose finding much greater profit in the cheap and tiny speakers. There are several things that make the 901 series speakers so special. The unique Direct/Reflecting design of the speaker causes most (89%) of the sound produced to be directed to the rear of the cabinet, away from the seating position. This sound bounces off the back and side walls before reaching you. The use of nine identical 4 1/2 inch drivers, rather than the more traditional big woofer, little tweeter, and just right midrange cones is another design choice. Nine small drivers, acting in unison, have about the same air-moving capacity as a single large woofer and mid-sized midrange cone. On the other hand, it's tough to get a 4 1/2 inch driver to put out high frequencies. And so here is a trade-off. Using multiple, identical drivers allows Bose to totally eliminate a passive crossover network, but instead, a special, purpose-built Active Equalizer is needed to extend the frequency response of the speaker unit.
Bose 901 active equalizer should go with 901 speakers. It is contained in a small box that is powered by normal household current (120 volts AC, at least in the US and Canada) and connects to your receiver or preamp by plugging into the tape loop or external processor jacks using two pair of interconnect cables. The equalizers are somewhat specific to the speaker series model--a Series IV speaker should use a Series IV equalizer. The 901 Series VI owner's manual says the Series VI equalizer is also compatible with the Series V. A Series II equalizer seems to be compatible with Series I speakers. It can be used with an antique Bose receiver, which will have an equalizer for an older model 901.
The 901 Series III and 901 Series IV have three speaker cable terminals. This third terminal is unused (and un-needed) unless you have one of the special antique Bose receivers. This third terminal is used as part of an effects device. This third terminal disappeared on the Series V speakers, by that time Bose had quit producing the special receivers.Aside from a Bose 901 Active Equalizer, either a stand-alone unit or integrated into a Bose receiver, you must be concerned about older Bose 901 speakers having problems with deteriorated foam on the actual speaker drivers. Repair kits are available to repair speakers suffering from foam rot. It would be a labor-intensive job to re-foam the 18 drivers in a pair of speakers.
The Bose 901s were legendary for their ability to handle large amounts of amplifier power. For a long time, Bose claimed that the 901 could take unlimited amplifier power, providing that a fuse of a specified value was inserted into each of the + speaker cables. This was back in the days when a 120 watt receiver was considered hugely powerful. Today, watts are cheap and readily available, and Bose is now claiming that the latest 901 (Series VI) will handle up to 450 RMS watts per channel, or 250 watts as tested by the IEC method . The 901 design produces big sound even with low-powered receivers or amps. They are very versatile with regard to the amount of watts you have available to feed them. The Bose 901 is small and (relatively) light for a quality speaker, easily portable, handles massive power reliably.
A properly functioning pair of 901s with the active equalizer is better than what is in 95% of the homes in America right now--primarily because there have always been a huge number of cheap-junk speakers being sold.The 901 is probably at its best when used as a very high-quality public address speaker system, or when used to provide sound reinforcement for such music-intensive public venues as skating rinks and teen discos.
