WOW! I've owned every model of the ESS AMT series speakers, but these were by far the best! I did not care for the earlier versions which always seemed to suffer from poor integration of woofers to the remarkable Heil driver. Such is not the case with this very late version offered by ESS just before they ran into internal problems and quality control dropped off. The fit and finish are the best of the series, and even the grilles are better,with a frame made from lighter synthetic materials then the particle board used in all the other models.
The crossover design for this model was outsourced and is "in-phase" unlike previous versions. The crossover point was even lower too@ 700Hz. instead of the 800 to 1000Hz. used in the AMT-1, AMT-1A, and AMT-1B.
The Heil fundamental resonance in this model was supposed to be around 350Hz. which is much lower then the Kapton ribbon units. Woofers are either a Bextrene or a Polypropylene cone 12" in diameter with a VERY heavy cast aluminum frame and a MASSIVE magnet structure. Added to this is a 12" passive radiator on the back, but for best room coupling this needs to be fairly close to a back wall. Away from a wall and the low bass will be reduced and you'll want to use a sub woofer to fill in the extreme lows. Not needed if close to 2 or 3 room boundaries.

The sound:
First,- it's is well integrated from the woofer to the Heil. Once you have the trim controls set for YOUR particular listening situation. A good starting point is 12 o'clock on both midrange and treble controls. Adjust from there for what sounds best to you. Another thing to think of with regard to placement is to consider adding a decorative rug on the back wall behind the speakers. That will reduce reflections from the Di-pole(radiating sound from the front and the back) Heil driver, a common source of complaints about the speakers being too bright. In all cases, it's the controls being set wrong, and/or errors in placement. This may not have been the only cause with the earlier versions of the AMT-1's, but by the time the 1C's were designed, the response was massaged into velvet smooth and silky highs. I did discover a simple mod that makes them even smoother, but I'll hold off on that for now.
Second - the bass is tight, and reasonably extended. Again, placement is of paramount importance here!
While the term "fast bass" is an inaccurate term, as woofers do not make "fast" sounds, they produce long wave lengths where speed is not a factor, still FAST is what comes to mind. When it comes to bass tones "fast' is really a matter of overhang, the ability to STOP and that comes from well designed suspensions and electrical damping as well as a correctly designed bass enclosure for the driver being used. So, though we can't properly say the bass is "FAST", We can say the bass in the ESS AMT-1C is solid, clean, low distortion, and properly damped. It has gut pounding thump when called for while never being over bearing in smaller bass heavy rooms. Most all speakers will benefit from a good sub woofer system, and that is true here too.
Third_ the Mids! The mids come from two drivers in this speaker. The lower mid range is handled by the Woofer, and it does a surprising job! I say surprising, because this was exactly the range of frequencies missing or not handled particularly well in the early designs. It's one of the best ranges for this system! The lower sounds of instruments emerge well defined and with body and lush bloom. Tom toms have impact and weight, and male vocals the correct amount of chest resonance, without being "chesty". This range then smoothly integrates into the upper mid range produced by the Heil driver. In brief, it's fabulous! Absolute clarity, equal to anything you'll hear from today's most expensive modern designs in any showroom. Small percussive strikes, like from fingers tapping on bongos are more realistically presented then you're likely to hear from any other speaker, while female vocals have an airy vibrancy which just emerges like it's coming right out a performers mouth in the room with you. Plucked instruments, also Cello, Oboe and trumpet and violin, all stunningly revealed with all of the brash energy of the trumpet, the silky sheen of the violin and the smooth alive sound of the others! Rock, and electric sounds? You bet, and then some! Here FAST does apply, as the highs and the mids are being produced by the same driver, and assuming you got those controls set properly you'll understand why they referred to this in the advertising literature as, "Sound As Clear As Light!" Absolute clarity!

Now, the mod. There is a SLIGHT response glitch caused by the way the metal magnet plates are held in the plastic housing. It is a small thing, but at high frequencies even a small protuberance such as a non flush screw used in mounting a tweeter can result in a response ripple. Here, the Heil driver has such a discontinuity from the magnet to the plastic housing. Easy fix - 1/32" thick self stick felt sheets, available from any craft supply house. Cut a piece just large enough to go from the inside opening of the magnet plates, where they are stacked solid together to and around the front of the plastic housing. Do this on left and right, as well as on the back. That eliminates the discontinuity, and the response glitch altogether. Many tweeters come with foam, felt, or in some cases a kind of velvet material already attached to their flanges, and this is for the same purpose as offered here - probably would help most of the ESS Heil models too. For .$.79 a sheet, try it, you might like it!

Other speakers I own and or have owned include :Magnepan ribbon speakers, Martin Logan electrostatics, Dahlquist DQ-10, 20' and 30"s., B&W 803's, and even a set of Monster Klispchorns! As well as many others. The ESS AMT-1C model speakers fit right into that group, and at today's price for a used set, are a killer bargain if you can find someone willing to part with theirs! The ESS Great Heil driver is still being made, and will set you back around $700 or more, for JUST the Heil driver. So a used complete system, with everything already worked out for you and for around the same price is a no brainer in my book.

Last note - This speaker SINGS with tube amplification. I have solid state and tube, and some speakers seem to work best with solid state electronics, and some are better for tubes. These things sound good with solid state(quality solid state) but absolutely come alive in a very special way with tube gear. Once you try them paired up to a good tube amplifier, and here moderate power is ok because the system is fairly efficient, you'll be in audio nirvana!