I got a nice TS 5998 a few weeks ago when a bunch popped up on ebay and have been busy listening and trying to match up 12au7s to it for a while now. I had a bit of an epiphany yesterday... I got a really cheap deal on some tung-sol 6080s (just plain 6080) a while back being the only bidder on an auction from Germany, had installed one and really liked it, but then started with the 5998.
I went back to the 6080 yesterday and... I prefer the sound over the 5998. It's got much more heft on the bottom end, the stereo imagery seems more "wide" somehow, and the top end is less fatiguing. I received two 1964 GE 5814A's today (this ebay habit is getting ridiculous but I couldn't resist trying a valve made one year before I was born, and they were only £13 each) and the match is wonderful, to my ears.
I do know I have a preference for low/mid and don't like too much piercing treble. Perhaps this combination happens to suit my ears, or maybe my 5998 is lacking somehow... or is it a case of the Emperors new clothes with some of the pricier valves? I was wondering whether anyone else ended up settling on something else having tried the 5998 or am I the only one?
Paul.