Bass reflex was the only new technology for a loudspeaker back in 1930 when people had 3 watt amps, and AM radio with 30db signal to noise ratios. AM only broadcasts down to 120hz. FM broadcasts down to 80HZ. It was hard to find the resonant problems with bass reflex when there was no true sub bass at that time. By the sixties the bass reflex lived on in folk lore and kept alive by parts sellers who knew it was a failed technology. fedloan irs.gov easybib