Pubul57, I am also challenged technically, but here is what I know (or, I should say, what I think I know...any of you EE types should feel free to correct me where I go astray): The type of core (EI vs. C in this case) simply describes the shape of the ferrous core in the transformer through which the music signal is effectively transfered from the primary windings (input) to the secondary windings (output) as the result of magnetic inductance. (The signal flowing around the core on the primary windings induces a magnetic flux in the core, which, in turn, induces an electrical signal on the secondary windings.)
With respect to the transformers that Promitheus is using in their TVCs, the EI-core transformer is rather small with comparatively thick core laminations (.35mm). The C-core transformer is much larger and heavier and uses finer laminations in the construction of the core (.05mm). The result is much better sound quality in the bigger and more expensive transformer. The flagship model TVC is the Signature model, which uses a pair of the C-cores in each transformer (one transformer per channel) and also uses more expensive Elma switches as the source selector and volume control. The sound quality of the flagship Signature TVC is significantly better than the EI-core models, which, in and of themselves are very good (and an excellent value).
With respect to the transformers that Promitheus is using in their TVCs, the EI-core transformer is rather small with comparatively thick core laminations (.35mm). The C-core transformer is much larger and heavier and uses finer laminations in the construction of the core (.05mm). The result is much better sound quality in the bigger and more expensive transformer. The flagship model TVC is the Signature model, which uses a pair of the C-cores in each transformer (one transformer per channel) and also uses more expensive Elma switches as the source selector and volume control. The sound quality of the flagship Signature TVC is significantly better than the EI-core models, which, in and of themselves are very good (and an excellent value).