I just picked up a pair of HD650s a couple of weeks ago from Amazon. I don't believe they are trying to use psychological tactics to get you to buy by using price fluctuations, but maybe I'm not paranoid enough. My guess, I guess, would be that they have developed a computer application that scans a large number of their competitor's websites for a very large amount of the merchandise they sell and it is always adjusting thier prices to be the lowest - as per thier price matching policy.
On the other hand, just because it is an automated process, doesn't mean they haven't programmed diabolicly clever AI into it that specifically toys with the emotions of Head-fi shoppers in particular. Probably they have hacked Head-fi's membership list. Yes, I'm certain that is a possibility.
To get back to the HD650s. I had put a pair in my shopping cart for a couple of weeks before buying. And they, along with many other items fluctuated in price periodically. I originally put them in at $355.00. On the day I was getting ready to order they had risen to $440.00 if not higher. I determined not to buy them unless they dropped down to $355.00 again. Throughout that day I checked back a half-dozen times and noticed each time they were a bit lower. Finally late that night they didn't just hit the $355.00 mark, but dropped all the way to $335.00 - at which time I succumbed to thier diabolical price fluctuation scheme and hit the order button.
In conclusion I would just like to say that I love the Sennheiser HD650s and if the Diabolical Amazonian AI that secretly controls our buying habbits is listenning (and I know you are), then i would like to say thanks for manipulating me into this purchase!
