Hooking up sub

Hi, I'd like to hook up a powered sub (Energy ES8) to an older Technics HT receiver. There is no sub output on reciever. I was told not to use the "B" speaker hookup as this could screw up impedence. I was thinking, could I hook up to tape RCA outputs Get an adaptor that merges the L+R into one RCA cable since normal sub output is mono anyway? If not what would be the best way to do this? The sub is powered and has a volume control and a crossover selector knob. By the way will be using 6 speakers, the front and rear L+R, center and the sub, all Energy speakers.

Thanks,
Tom