Class-D wattage instead of doubling for each halving of impedance load actually gets goes backwards when they see 2ohms and worse if there is any - phase angle involved, because they can’t deliver the current like a good bi-polar linear Class-a or a/b amp can.This statement is false. The limitation of a class D amp is similar to what is is with any amp- how big are the heatsinks, how much current can the output devices handle, and how much current is available from the power supply.
The output impedance of any modern class D amp is only a few milliohms (the Purifi module' output impedance is lower than a foot of 18ga wire). So it can double power from 1 ohm to 0.5 ohms as long as the current doesn't exceed the limits of the output section. The real issue is the filter; lowering the load impedance lowers the effective 'Q' of the filter, IOW it flattens out the filter response. So as the load impedance is reduced, the 'residual' (which is the sine wave remnant of the switching frequency) will increase in amplitude. For any speaker this isn't a problem as the residual is very small on any competent class D amp.
One thing I find reprehensible is the way the big names have added all kinds of tweaks to Putzeys’ design (op-amps, power supplies) that are completely unnecessary, or even cause spec deterioration, so they can charge more.The reason everyone has different input circuits is because the module itself doesn't have one. Its input impedance is about 2,000 ohms- most preamps and sources would fall flat of their face driving a load like that. When you're designing a module, there is only so much gain you can get out of it, and you have to use an input circuit called a 'comparitor' which tends to have that low input impedance. So some sort of buffer is typically needed. You are correct that this can color the sound if not implemented correctly. Bruno of course has a recommended circuit, but its not cast in stone that it be used.