Take it up with the likes of D’Agostino, Curl, ect ect manufacturers of the greatest amps that are recognized as being able to drive speakers with loadings that make Class-D’s run for the hills, they will tell you the same thing.Actually they won't. They will all tell you the same thing- the limit of any solid state amp, class D or no, will be current the output section can withstand, the ability of the heatsinks to adequately cool the devices and how much current is available from the power supply.
I will note one thing though- I've seen a number of SMPSs (Switch Mode Power Supplies) used in class D amps. I think its a bad practice. They work, sort of; the problem is that class D amps go from almost no current draw at idle to quite a lot at full power. SMPSs don't have a lot of capacitance at their output and so can be drained faster than they can replenish. A lot of them will shut down if you put a lot of capacitance at their output. So if you're not running an amp with a regular power transformer and a lot of capacitance available for the class D amp to feed on, you're probably not hearing what it can do.