I'd recommend a 1kHz crossover point for the 811B. You can also use a 1.2kHz or 1.6kHz crossover, each of these are common crossover points. Your choice will likely be dictated by your compression driver and your midwoofer.

The Altec 811B is an excellent horn, one of the best. One problem that you can run into is bell mode ringing around 1kHz. Mounting on a baffle and applying damping materials helps. Another technique is to cut the welds to destress the horn, similar to cracking a bell to prevent it from ringing. But in spite of these things, the Altec 811B is a great horn that provides great on-axis response.

As for midwoofers, I really like the Eminence Omega 15 and think it's an overlooked driver. It's not what I'd suggest for low-power tube amps, but for a more powerful system, it's an excellent part for the money. Response is good and it sounds nice.

On the other hand, the Omega 15 is not really in the same class as the 2226, because the JBL part is flux stabilized. The Eminence Magnum 15HO is a better comparison with the JBL 2226. The Omega 15 sounds very nice, but you can really hear the difference from a flux stabilized woofer. I'd say that of the woofers that aren't flux stabilized, Eminence has done a fine job and has some of the best parts of that type. But the difference afforded by the shorting ring is immediately noticeable. Woofers with shorting rings generally costs twice as much though too.