Don't mix, at least not like frequencies. If needed you could by with using the duals to cover 40-60hz, and horns to cover 60-90hz, but I wouldn't unless in a pinch. There is just a lot more going on then just delay, harmonics and phase are going to be all in diffrent areas causing untold cancelation, coupling, and assorted mud. It may be loud, but at the cost of a tight system.

CV is in an odd spot these days with Statton as the new owner and repair support is rumered to be sketchy at best. If you need a long hard life out of these there may be better options for the $.

Also mind that most hornloaded boxes, regardless of maker, are designed for 4-box arrays to acheive the mouth size of a 1/4 wave subwoofer frequencies (unless you have a nice corner handy). Other then some of the really exotic ($4k+) subs, most any hornloaded bin under 300-lbs is going to perform similar to a tiny sealed driver sub below the horn loading frequecy, which is typicaly arround 60-hz. Four together mate up to acheive the full sized horn (that would a wee hard to transport in one piece).

Some other boxes to look at are the Yorkville UCS1 ($600) or EAW's LA400 ($1000 - but maintains high resale value). Again, you need to have four to match the range of a typical dual-18" sub (of course you would have far more headroom and take a fraction of the power) in an open envioroment.