Many years after the original posts, I can say yes, it is worth upgrading the tweeters. I got my pair of Rec III highboys in 1975 from a grad student in Engineering Acoustics at Penn State. We went into one leg of the tweeter sets (there are two legs, each with 2 tweeters), put in an 8 Ohm resistor in place of the larger tweeter, and put a Phillips-Norelco 3/4 inch soft dome in place of the smaller tweeter on the top. This improved imaging and high end considerably, verified both by ear and in an anacholic chamber.
The original midrange is also a Phillips-Norelco, contained unit, with a whizzer cone. One of those had blown out, and back in the day, McKee made a respectable replacement. Only now 2018, am I really looking for replacement speakers. They are just a bit tired.
The Rec III was the first scientifically designed speaker, solved properly as a 6th order Butterworth box. These days, there are better solutions, but I've been reluctant to spend money on speakers that sound only a little better. Note that I have a large listening room, and many good sounding modern speakers do not move air like the big woofers and bass-reflex port.
The original midrange is also a Phillips-Norelco, contained unit, with a whizzer cone. One of those had blown out, and back in the day, McKee made a respectable replacement. Only now 2018, am I really looking for replacement speakers. They are just a bit tired.
The Rec III was the first scientifically designed speaker, solved properly as a 6th order Butterworth box. These days, there are better solutions, but I've been reluctant to spend money on speakers that sound only a little better. Note that I have a large listening room, and many good sounding modern speakers do not move air like the big woofers and bass-reflex port.