I own a pair of vintage (1987) POLK AUDIO RTA11-T tower speakers. my question is whether all the woofers used in the POLK line at that time were the exact same or different. I realize they appear to look exactly the same, but did they use different resonant frequencies etc??....From all appearances they appear exactly the same, until I took mine out recently and realized my towers had 2 different part numbers on them??....Who manufactured these woofers and are there replacements still available??