You are right about Sennheiser, the 4xx series look and sound pretty poor , almost unrecognisable against a product like the HD598/650.
Sony (the worlds biggest selling headphone manufacturer) are the same, they have a few pretty good cheapies , 570lp comes to mind but on the whole everything below 7506 level is average at best.
I dont think Hifiman are any different from the others as far as business practice goes , they offer good products at what seems like a great price but they are making their money on each pair sold, probably quite a good margin compared to the big boys.
Skullcandy are now officially the second biggest selling headphone manufacturer in the world, scary eh!
I see Skullcandys every day, on Emo chicks to businessmen in suits.
I think they sell well due to the colors they come in, simple.
I have a book on the Sony Design Centre and it has a bit on headphones, they had some cool concepts, one was headphones with changeable drivers for different genre's!