Lowther DX55
Comments: The DX55 probably has the best treble detail of the drivers tested and also enjoyed some of the biggest improvements being driven by a current source. I consider it the best single excuse to use a current source and it needs little or no additional damping in the bass, having but a small peak in a sealed box. It ended up being the driver of choice for the Kleinhorns since the horns provide enough low-end extension to make up for the diminutive size of the cones while keeping the excursion low. It is not the least expensive Lowther driver at $1095 /pair, but I am under the impression that the C55 will get most of this for you at $595/pair, making it more accessible. Keep in mind that subjectively, you will likely want R1 at a higher value than on the table, probably between 4.7 and 10 ohms.

Lowther DX55 in the Mini-Medallion enclosure
Comments: Lowther America was kind enough to lend me a pair of mini-Medallion enclosures, a relatively small rear-loaded horn designed to be placed against a wall. Used this way, they give significant extension below 100Hz, peaking at around 50-60Hz. The mini-Medallions have a bit of a horn peak at 200Hz. When the current source augmented the bottom end, I found myself putting about ½ lb of Dacron in the mouth to damp out the horn's pipe resonance. (This is the series of peaks that you get when the mouth of the horn is small compared to the wavelength). The upper left curve with the current source uses that damping. With some network trimming above about 500Hz, the sound was very satisfactory. Again, figure on a real-life R1 of 4.7 to 10 ohms.

Lowther DX55 in the KleinHorns
Comments: Just as corn chips are simply an excuse to eat salsa, Lowthers provide a reason to build big back-loaded horns. These specimens are about 20 feet long and 10 feet high, with a 30Hz taper and a 30sqft mouth. The measurements were taken at 6ft because the horn doesn't develop pressure in the center until you get a ¼ wavelength away. Between the 6ft distance and the mouth size, expect a roll-off below 50Hz, which can see here. Farther away, the bottom extends to 40Hz. The network for the KleinHorn is different from the others in that L0 has a non-zero value to produce the low frequency shelf attenuation. Otherwise, bass below 100Hz will overpower the midrange and treble.