Yeah, unfortunately, the simple nominal impedance rating doesn't tell the whole story. There are some 4 ohm speakers that dip way down to crazy low impedance, like 1 ohm and such. Then there are 4 ohm speakers that basically stay right around 4 ohms at all frequencies.
Pretty much any half-way decent receiver can safely drive a speaker that never dips below about 3 ohms. In order to get any level of THX certification, a receiver/amp has to be able to handle 3.2 ohm dips. From what I know of the Behringers, even though they are a 4 ohm nominal speaker, they are not a particularly tough load for an amplifier. They don't dip crazy low and they are fairly efficient.
In the case of the Axiom M3v3s that I've recommended, they are definitely not a difficult load for any receiver. They're an 8 ohm nominal impedance with a minimum impedance above 7.5 ohms and they are very efficient at about 92dB output at 1Watt-1Meter. Part of the reason why I recommended them is because the room size is medium and the seating distance is likely more than 9 feet away. That's going to require healthy output capabilities, which the M3v3 can deliver, even with a very moderately powerful amp. The Denon 1911 is more than up to the task of driving the efficient Axiom M3s
