The $30 (likely price mistake) SRH750DJ just arrived, and I'm blown away. Out of the box, there is excellent accuracy in the music I've listened to (thus far, some grimy hip-hop and some avant garde metal - not death) and the bass is super, super refined. Nice and punchy.
All in all, I think the seller did the mature thing. They fulfilled, as best I can tell, the orders of their stock, and are cancelling the people who backordered, which is probably the most we should expect on a price mistake and, provided everybody else gets theirs, I don't think anybody should leave them bad feedback.