A round soundbox is less common, a hexagonal or octagonal soundbox is more common. But it depends on the region. As far I know off a hexagonal soundbox will give a bright and sweet tone, a octagonal soundbox a deep and warm tone. Erhu with a hexagonal soundbox are common in south china while octagonal soundbox are common in north china. It's the same with Guzheng. Guzheng from the north are deep and warm. Guzheng from the south are bright and sweet.
My experience is indeed contrary to yours, hexagonal soundbox one has a much mellow result while may be a little thin, and has a better response than that of octagonal soundbox one. The octagonal soundbox one may have something like being "dry".