I recently bought four NOS RCA 5Y3GT rectifier tubes from Doug's Tubes.  Prior to that, I used to use NOS RCA 5U4GB's on both my TABOO and SE34I.2+.

With the 5U4GB, TABOO (listened through an Audeze LCD-2) lacked a bit of bass and the mids sounded slightly muffled.  Despite the fact that I still loved the sound of the amp, I wanted to give a try to the NOS RCA 5Y3GT, upon suggestion from a reviewer on head-fi.org.  

The 5Y3GT has made the already great sounding TABOO reach another amazing level of fidelity.  The bass has become much more prominent and tight, yet more balanced in comparison to the 5U4GB.  The mids opened up significantly and instrument separation is much clearer now.  The sound stage is higher and deeper, and the slightly muffled mids have left their place to a non-compressed yet analog-like warmth.  I listened to the system only for half an hour or so, but it felt like the fatigue from the cans would be much less.

By the way, both lucid modes were off during my listening session.  The other tubes are the stock 6N1P and SV83s that came with the amp.  

Next will be listening to the SE34I.2+ feeding a pair of Rega RS-1 speakers with the 5Y3GTs...

I find that 5Y3GT work best in my CSP2, CSP2+s and ZP3. . . just most natural and rich sounding, and I use recordings I made myself of bands I played with and recorded at the close of the 'eighties as my reference fof these things---I know the sound of the room and instruments etc.

I've settled on RCA being the best to my ears, at least in those I've used the last two decades, and the oldest one I have seems to sound the best. I hope it keeps going and going.