I use WE 300B's and have not tried the new Svet's. As I said, I was only reporting a shootout/review that you can find in the StereoFool archives. In my SET amps I've only tried the WE's, some Chinese (not unlistenable, but in comparison to WE's an entire world away), and a pair of EH 300B's...not bad overall, but just did not have the mid-magic vocals realism of the WE's and had a kind of edginess to them in the highs in comparison. As I recall the WE's just seemed more balanced too. I am very happy with the way the WE's sound so have not found the need to roll those tubes, perhaps much like your conclusion with your GEC KT88's. If money were no object to me, I'd roll away, and go for the best available tubes overall. But our money tree is very small, hence the cautionary point of view that one should weigh their priorities and investment against diminishing gains. In the case of the KT88's $300 buys a whole lot of music...well, gosh, I guess these days it'll get you a few CD's anyway!
Good input from everyone here, IMO.
PS Have you rolled your GEC's against the JJ Tesla's? I ask because I did that with a pair Quicksilver Silver 90 (pentode) amps and could not hear much of a difference to my ears. I don't think the tubes were Gold Monarchs though (at least I don't recall them being those, but this was years ago).