Oh boy, an area where I have some actual experience:
I've used some of the above mentioned tubes and can offer these observations:
The Amperex USA White Label 7308 tube was dynamic and textured with good bass response. A good tube but seemed to have a high frequency ceiling and seemed somewhat closed in compared to the Siemens. The bass was good and though perhaps not as tight as the Siemens, it had substance. One day I may want to give this tube another try as no one seems to agree with me on it's sound.
The Siemens CCa sounded as most Siemens sound: linear, crystal clear, transparent with tight transparent bass. I'd agree with coolish, perhaps slightly antiseptic over-all. The midrange has an almost solid state quality but slight. Don't get me wrong, this is a tube you will want to keep in your collection. What it does, it does like no other and I would put it in the first position of amplification. Go with clarity up front (like your source), because once you've lost it you can't recreate it further down the line. Do you know what I mean?
The Valvo E188CC red label (which I have heard was their premium line) was a marvelous tube: powerful, dynamic, great bass liquid mids (relatively speaking) and extended highs. More colorful than the Siemens, I'd say, like the Amperex but better overall.
The Telefunken: Be careful of this tube <> on the bottom or not. Telefunken produced tubes that had two totally different sound characteristics. 1)Warm and soft to the point of putting you to sleep and giving tubes a bad name and 2)Neutral, extended response with deep and tight bass and the most natural presentation that you are likely to hear from any tube. Neither version by any stretch of the imagination,like a Siemens at all as is reported by a popular tube vendor. Unless you have really have done your homework, and I thought I had done mine, I would only refer your purchase of this tube type to an expert. Maybe a German vendor.
I ended up with a set of Ei 6DJ8 A frames, which is supposedly a copy of some famous Amperex or Siemens. Gorgeous! I think the reports of Eis going microphonic is a lot of hooey. One day (after the recession) I will purchase a pair of Siemens E188CC which I feel may have a bit more body than the CCas or if I can id the real deal Telefunkens, a pair of those.
