OK, so I went ahead with a pair of sealed (pine plywood), 4 ft^3 cabs for my UM-18s. They sound pretty good, but I'm getting a sharp dropoff under 40hz (see attached graph). I don't think it's a problem with placement, since the graph shows both subs with the same pattern.
Did I make a mistake going with sealed? I've seen others getting strong response in the 20-40hz with the same setup. Or maybe Audyssey is doing something to limit 20-40hz?