A phase plug would extend the upper response limits but I only need output to 2kHz. It is fine for the path length differences to start rolling off output above that because the cone starts entering breakup around that point anyway. What I'm finding is response to 1kHz is pretty easy but it's harder to hit 2kHz, especially with larger drivers. But I don't need output much above 1kHz anyway, just enough to sum with the tweeter as it comes online.