I have been retrofitting E-Horns and W bins for more than 25 years.
The Definimax 18" certainly is designed for horn loading, as was the original Omega driver before they changed it completely in it's redesign and the appropriate parameters were designed into the Definimax. The original Omega had vastly different TS parameters than today's two versions. Those parameters included a Bl factor in excess of 25 Tm. I used lots of them. The newer versions of the Omega from the arrival time of the Definimax, were designed for Bass Reflex enclosures.
The Definimax 4018LF has an Fs of 32hz, a QTS of .35, a Bl factor of 27.0 Tm, and a ribbed cone to add rigidity in order to facilitate the compression of air, thus forcing it through a restricted throat area. Manufacturers do not design in high Bl factors for any other reason, that I am aware of. A ribbed cone is indicative of a woofer designed for compression loading and it is not a requirement in Bass Reflex loading.
The ideal woofer for horn loading should have a Bl factor or "Shuv Factor" in excess of 25 Tesla Meters. 20 Tm is not enough motor strength to withstand the rigors of compression chamber loading. Ideally the QTS should be less than 30 as stated previously and the Fs should also be in the low 30s.
That said, variance slightly with regards to the QTS and Fs are tolerable if under 40 in either case. There is a likelyhood of ringing occurring if the numbers exceed that. 25+ for the Bl factor is a must.
The biggest bang for the buck in a specifically designed woofer for E-horn replacement, (which technically is more of a variant of a Voight Pipe than an actual Folded Horn),is the Selenium 18WFH1P-SLF. The W stands for W-bin and the FH stands for Folded Horn. Selenium makes this woofer and it's variants that include the WFH2P and WFH3P, which are not readily available in this country.
The afforementioned 18WFH1P-SLF is available at Parts Express and I have also used it many times with great success. It features an Fs of 34.5, a QTS of .37 and Bl factor of 27.7, and sells for $186.61. Furthermore, it is rated for considerably more power than 1000 watts program or peak.
http://www.parts-express.com/pe/pshowdetl.cfm?&DID=7& ;amp ;amp ;amp ;amp ;amp ;amp ;amp ;amp ;amp ;amp ;Partnumber=264-398&ctab=2#Tabs