A conical horn has no more problem with standing waves than any other horn. If the horn is flared in both axis, then standing wave nodes don't set up side to side, like they would in a basshorn having fixed width the whole length of the horn. That's where the problem lies. Basshorns with straight side walls and straight passages inside can develop standing waves along those dimensions if they're used up to high enough frequency. So it's not the expansion rate that causes notches in response, it's the constant width of the pathways, in a cabinet so constructed. If the horn isn't used to high frequency where the first standing wave node causes a notch, it's not a problem. Most basshorns are used only at low frequency, so straight side walls don't matter. But if used high enough, they'll cause notches in response.