I think you'll be just fine with the JBL 5" on a 300Hz tractrix horn. Is your Edgarhorn rectangular or round? Not that it matters for the crossover frequency or slope, but I'm just wondering. You have a rectangular 811 tweeter horn, so it seems to me that it would be fitting to have a rectangular midhorn. But the round horns are all the rage for most Edgarhorn enthusiasts.

Either way, I'd have to say that my first choice wouldn't be a combination of 811 tweeter horn and a 300Hz tractrix midhorn. I'd either use a smaller tweeter horn or I'd ditch the midhorn.

Your 300Hz midhorn will be capable of carrying you pretty high, probably to 3kHz or higher. So I'd probably use a different tweeter when using this midrange horn. The 811 tweeter horn is perfect for use with a midwoofer run up to 1kHz. So that's what I'd do with it. Make a two-way system for the 811.

The three-way system using a 300Hz tractrix midhorn and 811 tweeter horn will work fine, so if you have your heart set on this or if you already have all the components ready to go, then by all means, do it. There's no problem with the configuration. But you asked my opinion, and so I'm just telling you that if it were me, I'd run the 811's as two-ways crossed at 1kHz or so, and I'd use the 300Hz midhorns in a three-way system that had a smaller tweeter horn crossed over somewhere around 3kHz.

Wayne