Back in the late 80's, I picked up a pair of Yamaha NS-20M speakers for a song at a consignment shop. I've been using them for years with an old but reliable Denon receiver, but have not been able to find a lot of information about these speakers. Can anyone provide any info on these speakers (dates of production, original price, etc.)? They are heavy and built like tanks, and I suspect they were originally desiged to be used as studio monitors. Thanks for any thoughts you might have.