Bookshelf speaker series tannoy mercury power rating 40 watt effective output 160 watt system 2 way system speaker system 20 stereo principle passive frequency response 53 32000 hz sensitivity 87 dbmw impedance 8 ohm crossover frequency 3400 hz recommended amplifier power 20 80 capacity 47 litres power supply passive colour brown cabinet surface walnut design front firing subwoofer speaker enclosures bass reflex removable speaker cove