I just received my Teac HA-501 a few days ago. I wholeheartedly agree that it is a magical combination with HD 650's. I never want to come out from under the cans.
Did you try any other amps before the Teac? I have the AH-01 and really like the build quality and the DAC/AMP section is really good, a little on the warmer side which I like with my HD-700 and RS2e. I'm on this thread since I have a HD-650 ($290 amazon) coming in on Thursday or Friday and wanted to get another amp to drive it correctly. Been reading a lot of threads for the phones that I have and the amps that I'm interested in, the more I read the more alternatives I'm hearing about, talk about info overload.