I have put together a mockup of the top of the Smack production prototype. In this photo you can see the standard TRS stereo headphone jack mostly obscured by the 5687 tube at the front, in the same position as the S.E.X. amp. At the chassis rear behind the power transformer you will notice two XLR jacks. These will be for balanced headphone cables. There will be switches under the chassis to set the output impedance (16/32/64/128 ohms) and whether the output transformers are configured for balanced or single ended operation.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.bottlehead.com%2Fsc_images%2Fsmackbackweb.jpg&hash=8ccca74deb6340219a2fd9d459505339f8a9389a)
http://www.bottlehead.com/store.php?crn=224&rn=455&action=show_detail (http://www.bottlehead.com/store.php?crn=224&rn=455&action=show_detail)