Bookshelf speaker series tannoy mercury power rating 50 watt effective output 200 watt system 2 way system speaker system 20 stereo principle passive frequency response 42 32000 hz sensitivity 89 dbmw impedance 8 ohm crossover frequency 3400 hz recommended amplifier power 20 100 capacity 94 litres power supply passive colour brown cabinet surface oak design front firing subwoofer speaker enclosures bass reflex removable speaker cover