The (1987) C40D was a derivative of the C40 with one of the two active bass drivers replaced with a passive radiator, producing a design similar to the Cara(1986).
Just picked up a pair at the local swap meet for $55, and these things rock, and they sound great with the Sansui 9090db driving these, but what doesn't. Nice and warm sound with the right amp, not quit as warm with the Harmon Kardin 730, or the Marantz SR 840.

The Kef C40 is not a Passive radiator model. Both woofer/ mid are electrically connected to the crossover & input & receive signals. The crossover does send a slightly lower freq signal to the exact same model bottom woofer. So let's clear that bit up. Being a mastering engineer & touring factories & seeing designs made & developed, plus my hobby of rebuilding speakers gives me an edge. The C 40 looks great. But it has a 'bottom of the well' voice related to the crossover. Bass, unless from an upright bass & closely miked is very ordinary indeed. I doubt if its over 70hz & down in level, thus the crossover design to make it seem low..its not. In Aust $ they were ave $1,000 & thats expensive for the year they made. I've played with the crossover, its difficult cause the drive unit impedences are unusual. Putting a metalised 102 bypass capacitor on all the caps helped transients. Removing the caps that blocked frequencies to the bottom driver helped bass level a bit. Lining cabinet with rubber carpet underlay helped the midrange. But overall, not a good design, tho the components quality & build are excellent. Perhaps its the heavy rubber surrounds that inhibit bass or simply wrong drivers for an acoustic suspension system. Either way, they are tonally very wrong, sluggish, lacking in rhythm, even with the mods. I just rebuilt a 1970s AR 4x small 2 way accoustic suspension speaker back to original, cost $55 Aust new & it wipes the kef C40 low & mid bass...Go figure!! Shane Cook.

Re: Acoustic suspension and bass reflex are totally different types of cabinets.These are acoustic suspension.

I got a set of these from my dad. Its over 15 years old and my oh my, THE QUALITY OF THE SOUND IS STILL AMAZING! Its connected to a Yamaha RX 476 receiver.
Wow I`m impressed. As long as you use good quality wiring these speakers will give you a lifetime of excellent sound!!

Wouldn`t buy anything else than KEF.

Good mid range, voice is well represented. Highs are clear and crisp. Bass is strong for the size. Easy on the eyes( to me at least).There is some warmth and a very laid back sound. Bang for the buck is comparable to some of the best out there ( paradigm titans come to mind.....}

A big book shelf thats not going to make love to every ones eyes. Laid back sound is also not a trait every one favors and these are definitely laid back. Not the ideal front speakers in home theater set up. Finally, these speakers do get " confused' , if only slightly, during the most complicated of sections. One more note; Audiophiles do not need to apply if you think you're going to find the most accurate speaker known to man that compares to you're $50,000.00 set up, for gods sake , they are only a hundred bucks :)

First off I would like to state that I am not an audiophile, I do love two channel music, and I also love to shop all the local thrift stores and pawn shops and what-not, to find vintage audio gear.I can however differentiate between speakers and cables, and i can hear( usually) the nuances between certain formats. With that being said my audio experience has for the most part been relegated to American speakers. I am not going to be the elitist snob who is going to knock the domestic product, I feel that some of the best speakerst ever made has come from the good old U.S.A. Now with that statement behind me let me tell you when I first heard my Kef c40's I couldn't help but think,"wow!!!, the britts sure got there audio act together". These two- way speakers( two and a half way I have also heard them referred to as) with a single 1" dome tweeter and 8" active woofer and 8" passive radiator, really impressed me. The enclosure they provide is an acoustic suspension bass reflex design, which even though normally not the most efficient enclosure compared to a ported cabinet, are very modest in their power demands, and very capable of delivering robust low end in a small to medium- size room. When I first got a listen to them i definitely liked what i heard, but there is a getting acclimated period with these speakers. With American brands there is a up-in-you're face quality too them, or aggressive i guess, is the simplest way to describe them. With the kef's though, there is a relaxed air about them, almost like sitting next to the fire with a good book and warm brandy, and warmth and tonality that doesn't stress the ears, and detail that assures that all is represented along the entire musical spectrum. looks with these speakers are also a plus as well, though that is as subjective a matter as sound is, but with walnut veneered cabinets and a hight of 25" and dept and width of only about 9", these speakers should be wife approved universally.These speakers do need stands, when I did some research i found that out, and when i did find some( small end tables seem to be the best remedy), I was rewarded with an even larger more focused sound stage and a better representation of the lower portion of the frequency range than before. These speakers seem to excel in any genera of music, from classical to jazz and most in between( for rap I couldn't tell you, sorry that's not my cup of tea) , but it seems that these speakers really shine when you play some rock. They also are great for folk or any thing with small acoustic ensembles, like Bob Dylan or James Taylor, or small string quartets. I wish that I could tell you exactly the frequency range, but with most speakers, especially one's from the eighties, I believe it's in the 35hz-20,000hz range. I only mention this because high resolution formats like SACD and DVD-audio sound really good from these speakers despite the modest frequency response. Vinyl also is not left behind and is well represented with these speakers. So far I have only been complimentary of these speakers, but as with all things there are draw backs. For one thing, these speakers are not that big but they are not that small, and with the home theater craze with small speakers and sub woofer set ups these speakers will not appeal to every body, especially since speaker stands of the standard variety will not do, and as I mentioned earlier you will probably have to get some sort of end table( don't forget to put some sort of resonance- dampening material between the speakers and the surface of the table), which makes the speakers foot print even bigger. Speaking of home theater, I wouldn't suggest using these for front speakers in a theater set up, they are competent enough, but it just seems that's not their strongest suit. Finally, the only other draw back about these speakers is concerning music. they can handle most styles that you throw at them, but during the most complicated of arrangements, it seems to me, that they can get a little flustered. It wasn't a glaring fault, and should not let any one be dissuaded from giving these marvelous little bang-for- the bucks a test drive, and then, eventually, a home.

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Polk's and Infinities of all shapes and models and have auditioned some other brands, but I won't wast you're time since I didn't buy them.