I think with Class D it boils down to the chips they use, no? I'm no expert on the subject, but in the case of NAD per the youtube vid above, they use some whizbang Hypex module from Norway (I think), they're leaders in the space from what I know - probably royalties they pay, etc. Thus in the NAD example or PS you'd get the real McCoy vs some Chinese chip in a lower priced model. Maybe the moral here is that in order to do Class D right, you need to shell out the bucks for the chips.
