It takes a lot of power to drive them. From memory each speaker has 9 of the same size and type small speakers (5 inch maybe) or drivers. 4 are mounted in each of the 2 virtical sections facing rearward and one is mounted facing forward in the front panel. There are no crossovers because all the drivers are the same. That is why the Bose equalizer must be used with the 901s, to adjust the low, mid, and high frequencies (the equalizer actually attenuates the low, mid, and/or high frequency ranges out of the turntable, CD player or receiver prior to being amplified by the power amp or stereo receiver amp) to sound correct. Speaker placement and room acoustics has a major impact on sound quality and one must experiment quite a bit to get optimal placement. I use an 125 watt per channel rms, 20 hz to 20 khz @ .001THD Onkyo receiver from the 1980s to drive them. Set them up right and they sound great.
I also have a pair of same vintage EPI Time Energy speakers I bought new (for about 1/3 the price the Bose 901s were) that used to really sound great, but I think their crossover capacitors need replacing and the foam speaker surrounds are failing.