Shows the ignorance of the dealer. I will say Mac can sound amazing with the right speakers and not so great with others.
They sound as good or better than anything with Klipsch and other horn loaded speakers, also with B&W diamonds, raidho, marten, spendor D series.
Not so great pairings would be smooth brit mons, the new wave of big box speakers like wharfdale and KLH, Audio note or DeVore.