Just to be clear, with many pairings I would agree with you. But my comment, which you quoted, was specifically in reference to the crack, in answer to roger's question about that amp. Hd650's sound particularly good with the mainline, to me, relative to how I perceive my hd800's through my crack. These are combos I have spent hundreds of hours listening to, and I thought a comparative statement might be of use to some in the thread under the hypothetical that I had to sell off either my mainline or hd800s. As indicated in my signature block, hd800's driven by the mainline are what I listen to every evening, and I want for nothing with the yggy as a source.
I didn't make any global statement or slam on mid-priced amps. In fact, if you go back in the thread, you will find I have previously commented that hd800's are plenty enjoyable right out of the HP jack on my marantz HTR, but I think most of us would agree that they do scale in a significant manner and sound particularly good with certain amps, if you have had the opportunity to sample a wide variety of what is out there.
YMMV is a truism as to any audio gear, but if you haven't heard the mainline, you really aren't in a position to say "nope." All the +1's and posts the piggy-backing on yours seem premised on a misreading of what I actually posted. Reading comprehension seems to be a lost art in these parts.