I currently have Promitheus Audio Reference C-core TVC and I can confirm that this unit is extremely transparent and extremely quiet. Soundstage definition and resolution is the best I've heard in any system at any price. Each instrument has its own well defined space. Dynamic contrast is top notch. This TVC doesn't have the midrange magic that tubes provide, but it gets pretty close.
This is the top-of-the-line TVC that Promitheus Audio makes, so I don't know how their other units compare to this one. From what I've read, these TVCs don't work well on systems where there is not enough gain. I don't have that problem at all. The sound is already too loud if I pass the 1 o'clock mark.
Regards,
iSanchez