Ok, that makes sense. I was really thinking in my mind all this time that pro amps were just a higher power handling sibling to consumer amps, but it makes total sense that a pro amp would be able to drive that load with lower distortion etc. Is it just a quality issue, or just designed that way?
Some of the reasons seem to be a matter of the intended market. It's typically an expectation in the pro market for a two-channel amp to be specified for bridged-mono operation into 4 Ohms. In bridged-mono operation, the amp in effect "sees" an impedance of one-half the actual load. So specifying the amp for bridged-mono operation into 4 Ohms is the same as specifying it in two-channel mode, with each channel driving a 2 Ohm load at half the power of the bridged-mono scenario. So in the case of e.g. the Crown XLS 1000, it's rated for 1100W into 4 Ohms in bridged-mono operation, and 550W per channel into 2 Ohms stereo. You'll see this pattern in two-channel amps that are capable of bridged-mono operation (per-channel output power into R/2 Ohms is one-half the bridged-mono output power into R Ohms).
Another consideration in output power capability is how hot the output transistors get for a given power delivered to the load. Two major contributors to this are the capacity of the amp's cooling system and the efficiency of the amp. Pro amps usually have cooling fans, and it's often the case that a cooling fan with modest heat sinking and efficient design of a "wind tunnel" to exhaust the heat is more effective thermally than a monster heat sink with no fan. This is at the expense of fan noise of course. A modest heat sink with a fan is less expensive than a monster heat sink as well.
Regarding efficiency, there are amplifier topologies such as class H and class D, often used in pro amps, that give substantial improvements in efficiency over the class AB mode typically used in consumer amps, at the expense of a slight increase in distortion. Higher efficiency means less heat needs to be removed from the output devices from the outset. The slight distortion increase of class H and class D designs over class AB is mitigated by the use of lots of negative feedback in the amplifier. Negative feedback does a better job of reducing low-frequency distortion than it does distortion at high frequencies, because of a technique called frequency compensation which is necessary to keep the feedback amplifier from oscillating. But for a subwoofer amp, we only care about low-frequency distortion anyway, so if the fan noise can be kept low, using a pro amp for a subwoofer can be a big win, providing high power at low cost.