If you are interested in the internal construction of the TX-102, you might have to buy one and take it apart to find out. Why don't you ask Jonathan Billington why he doesn't use double C-cores? He might have a very good explanation.
It's rarely true that one particular parameter or component is solely responsible for the sonic success of an audio design. I'm sure that a double C-core TVC could be made to sound inferior to an E-I core TVC.
Kevin Carter
K&K Audio
www.kandkaudio.com