My first stereo system, purchased after college graduation in--uh--the second half of the 20th century, consisted of an Eico ST70 tube integrated amplifier, an AR Turntable with a Shure cartridge, and Acoustic Research AR4x speakers. All of these components were Best Buy rated in Consumer Reports, and cost me a total of $277 delivered to Urbana, Illinois from a Washington D.C. discounter. The speakers were two-ways with an 8" acoustic suspension woofer, a cone tweeter, a series inductor for the woofer and a physically huge 20uf "oil" capacitor for the tweeter, working through a variable resistor notorious for its tendency to corrode. The best feature was the signature AR oiled walnut cabinet that defined "contemporary" back in the day. Consumer Reports described the AR4x's as "One of the smoothest, widest-range speakers we have ever measured, with a slight exaggeration in the midrange that lends a barely perceptible 'fullness' to the sound." I tried to like them all summer before graduate school, but traded them in for some KLH 6's because the AR's sounded like someone lowered a 1" thick velvet curtain in front of them.
Flash forward about 300 years. Someone who put together my Insignia mod offered me a pair of 4x's he had lying around apparently gathering dust. They turned out to be in great condition with just a few dings in the cabinet. The notorious AR tweeter pots even worked. I hooked them up and--it was velvet curtain time. Just as I remembered them. The measurements tell the story (which were taken with the tweeter control all the way up). My on-axis plot shows a huge peak at 1 kHz, followed by a cavity at 2 kHz, and then quite smooth response (ignoring the benign peaking in the high treble.)
The 1 kHz peak is due almost entirely to a tweeter resonance that the simple 20 uf capacitor high pass filter can't deal with. Here's the tweeter response with no crossover in line:
The 1 kHz peak blurs the sound and the 2 kHz dip adds to the lack of detail, but I would have expected a more open sound than I heard just judging form the tweeter performance higher up.
The response 30 degrees off axis explains the dullness. The 2" cone tweeter has truly awful dispersion.
I'll replace the tweeter with a modern unit, keep the woofer, and design an appropriate crossover. That should make someone a nice pair of speakers.