New to Class D / Purifi - thoughts before I purchase, details below

Thank you everybody.

@atmasphere 

The distortion level of a Hypex amp is 0.0006%.  That's already below audible levels, so how would the input buffer matter?

@skeptikal As I said, the buffer is likely to dominate the actual distortion of the finished amplifier. Hypex and Purifi modules do not have their own input buffers (although they do sell buffers separately).

With distortion that low, you're hearing how the buffer sounds, but if the buffer isn't good at its job, it can mess with the module and distortion can be quite a lot higher!

The module has an input impedance of about 2000 Ohms, too low for most preamps to drive. The module may not have enough gain either. So the buffer's job is to drive that 2000 Ohm load without distortion while adding any needed gain, and in some cases also allowing for a balanced input. 

I have to admit to being a bit disappointed at how bad some buffers are! For example, usually they use opamps. There are plenty of opamps that can drive a 2000 Ohm load, but you have to make sure there are no DC offsets at its output or the module can behave quite badly! So design of a good buffer is non-trivial. However IMO its a sad state that there seem to be a good number of input buffers are obviously not up to snuff. 

Its exactly on that account that we built our buffer into our module so that variable was eliminated.