I got a pair of Kef 104 speakers in 1976 and used them until the early 1990s. When the capacitors in one of the crossovers went, I had the newer 104aB crossovers installed. With the original 104, I preferred the midrange control set to the -2 dB position. With the 104aB, I preferred the +2 dB position. I sold them after I got the Quad ESL-63's, and as far as I know, they are still in use.
As TLS mentioned, by post-digital standards, the tweeter was not very robust, although it was tougher than many of that vintage. It should be OK unless you run high frequency test tones through it. Musical transients should give it no difficulty.
Another idiosyncrasy is that the passive radiator, which is a port substitute, does not load the woofer at very low frequencies. This could be a problem with record warps on LPs, at such low frequencies, the excursion of the powered woofer is not damped. This could be a problem at high levels, according to the reviewer in Audio magazine. Although the woofer could handle a lot of power above 100 Hz or so, it could not play cleanly if hard driven under about 40 Hz. The only time this was a problem was if I tried to turn up the bass on organ recordings—after a certain point, the deep bass simply would not get louder. If you get to that point, back off a little. But if you don’t do something like that, the speaker actually has a deep and solid bass response in a 12’ X 20’ room at ordinary levels.
The sensitivity is about 85 dB, I think, but the minimum impedance is only about 7 ohms. However, the speaker is fairly reactive around the crossover region, so it does require a fairly decent amplifier. I used a HK 730 receiver (rated 40, actually 50 watts) for years. I had ambitions to get Quad ESL-63 speakers, so when the receiver’s left channel blew and no one seemed to be able to repair it properly, I got a Quad 606 amplifier (140 watts), which I still have. I felt the larger amplifier was a little better for CDs.
The 8 inch woofer is crossed over to the ¾ inch tweeter at 3 kHz. This necessitates that the off axis dispersion is not very even between about 1000 and 2500 kHz. The speaker has a very even response within 15 degrees right or left of the tweeter axis, which Kef called the listening window. So, if set up well away from the side walls, and with the fairly prominent bass response, about 18 inches off the floor and not too close to the back well, they can still sound very, very good. They are superb on male and female vocal and piano, and quite good on full orchestra and chorus.
I found they sounded much clearer with the foam grilles in place. These damp the cabinet reflections. Unfortunately, they also deteriorate after about 10 or 12 years, and are hard to replace.
If you can set them up properly, you can get good results even by today’s standards. But they are not for head bangers. My advice is not to get something else until you find something you like better.
You can find the specification on the Kef site. Look under Museum, click on the 1970s, and then click on Model 104.
http://www.kef.com/ca/about/museum
