Bookshelf speaker series roth oli system 2 way system principle active frequency response 80 20000 hz sensitivity 82 dbmw impedance 6 ohm amplifier type digital housing surface veneer speaker enclosure bass reflex number of tweeters 1 number of mid rangetweeter drivers 1 mid range driver material midrange fibreglass weight 48 kg area of use home cinema hifi system living room installation options vertical shelf