Phil, that sounds like an intriguing design, and I'd like to see a schematic just so I can understand the system you're describing. One thing I do want to mention, though, is that even though an acoustic mass element is used in my design, there is very little (if any) helmholtz resonance taking place (the type of resonance where the entire volume of air inside the enclosure comprises the air spring). Instead, the mass element is there to modify the quarter-wavelength resonance of the pipe. As best I understand it now, it's like adding more path length to the waveguide, but more resistance as well. A constriction at the end of the horn, being of negligible length, would behave acoustically like a resistor, and would only serve to increase the resonant character of the horn while decreasing overall SPL response, but a mass element (such as a port with a defined length) has a reactance that shifts the low corner of the horn's response even lower in frequency.
Send me a PM with details of the design, if you'd be so kind.