(headphones: HD600) I recently got a Winged C tube in and paired with the RCA Clear Top 12AU7, it's blowing my mind compared to the 5998. The soundstage is significantly improved, and I feel like it's much much more musical compared to the 5998's linearity. The 5998 is more dynamic and detailed in general, but the Winged C is so much more enjoyable. It also responds to the input tube completely differently...
Listening with the 5998, my take on my 12AU7 tubes are:
RCA Clear Top: bright and revealing with a good timbre but lacking bass entirely, so meh (worth noting that the 5998/RCA combo with HD650 was absolutely jaw dropping)
Genalex Gold Lion: Awesome soundstage, great bass slam, rich mids, slightly recessed treble, spectacular for rock and metal
Amperex made-in-Holland: Totally natural through and through, sweet sound, the only choice for classical/jazz/anything well produced
Now with the Winged C, it's become:
RCA Clear Top: amazingly clear and revealing and dynamic, incredible timbre and soundstage I didn't know HD600 were capable of, no lack of bass at all
Genalex Gold Lion: meh, a little too mid-centric IMO
Amperex made-in-Holland: warm, smooth, and sweet... still need more time for more
The range of sound available from the Crack-HD600 combination is incredible! Not to mention that the Winged C/RCA combo is so absolutely one of the best and most lifelike things I've ever heard. I was finding it hard to get work done last night because I was just so breathless from how good everything sounded. Definitely too revealing for poorly produced music, although I'd swear the 5998/Gold Lion combo can make bad recordings sound like good recordings.
Still keeping my eye out for a good price on a Tung Sol 7236 then I swear I'm done buying (power) tubes for this thing and it's on to 6SN7 input tubes... when does it end?!