I was just reading an exerpt from Love Saves the Day: A History of American Dance Music Culture, 1970-1979 by Tim Lawrence. Mancusco used both Klipshorns and La Scalas in his club. You can regularly find La Scalas for 500 - 600 dollars on ebay, sans shipping. The bass is not as deep as the K-horn, but for modern music, you will have to augment the bass anyway. Finding an appropriately fast, high output subwoofer will be challenging. If money is not an issue, Avante-Garde makes a horn subwoofer. They do look cool, so I guess you could justify the expense becuase of the wow factor. They could be prohibitively large for your venue. While on Avante-Garde, their main horn speakers are highly regarded in audiophile circles. Keeping them safe and yet visible so the MOMA design can be admired could be problematic. Shahanian Acoustics made a large horn subwoofer called the Contra-bombarde whose form factor is friendlier, but I have only ever seen one for sale. There is a pro grade servo driven sub made by ServoDrive inc. They have two models, a compact bass reflex (ContrBass) that goes down to 18hz, and a horn model (ServorDrive BassTech 7) that goes down to 28hz. Given the small space the bass reflex model probably fits your need, though it is not as efficient as the K-horns. You may even be able to arrange getting one to audition in your space. VMPS makes "The Larger Subwoofer" that has similar specs to the ContraBass, and is under $1K, though may not be as bulletproof.
