Headphone tube amplifiers and OTL

Anyone have personal experience with this growing segment of the audio world. Am interested in learning about OTL and tube headphone equipment. Antique Sound makes some units. Any Good????? Are the Sennheiser phones a good match???

Granite Audio had a tube headphone amp on display at CES this year, however, I don't see it on their website. It had a wonderfully sweet sound. You can see it on the left end of the table in this picture.

Yes, I have an ASL MG Head OTL tube headphone amp, and it sounds fantastic when matched with my Sennheiser HD580. Check out www.head-fi.org, a great headphone resource, particularly the amp forum, where there is lots of discussion about the MG Head, and lots of anticipation towards the upcoming Mark III version.
Also consider the Berning micro-zotl which is an OTL/tube design that can also function as a one source integrated driving small monitors or other high eff. speakers. I use mine w/HD-600s and also to run some Sequerra monitors and it is very nice.
The Berning is a nice amplifier but in no way can it be considered an OTL. Just FWIW.
Well I guess I stand corrected. Not sure I understand the diff btween zero- hysterisis and otl, but it sure sounds fine.
Another excellent option are the Singlepower headphone amps. I have one and love it. Some guys over at Head-Fi feel that these amps are among the best SET/OTL headphone amps available at any price. They're at 6sn7.com Go to the Maestro link and scroll down. There are several pictures of different models on this page. Good luck.