Nice summary of brands and house sounds. I concur with a least one minor difference. I found a black plate RCA to be a real surprise performer. My main rig is a Willsenton 23 wpc SET integrated amp. (I know, ChiFi haters, but it stacks up very close against my Cary Audio 300SEI LX-20 running KR 300B XLS super tubes.) The pre stage is a single 12AX7 signal tube followed by a pair of 6SN7 gain stage to a pair of 300b driving 845 output tubes. Speakers include Klipsch Forte IIIs and Focal Aria 926. Source is a fanless silent mini-PC feeding a Denafrips IRIS DDC to a Denafrips Pontus II 12th. Dedicated power line with true earth ground and hospital grade pure grade copper AC outlets.
The amp is hot, meaning more than enough power to drive the high-sensitive Klipsch Forte's. Volume pot 1/4 or less, probably only using 2-3 watts. I often swap out the 12AX7 for lower gain 5751s and 12AT7 when listening to the Klipsch. The Focal's handle the full gain 12AX7s just fine. I have the same NOS brands as Jimmy2615 for 12AX7s and 12AT7s. 5751s are GE, Sylvania and RCA.
GE = good overall balanced linear sound, nothing distinctively great, nothing distinctively bad. "Workhorse tube," good value.
RCA = very warm smooth tube with great detail. Black plate 12AT7 is one of my favorites in the map, particularly with jazz, folk and vocals. Great value for performance. Suprise performer, I paid $12 for it. First place for a relaxing all day non fatiguing listening experience.
Sylvania Gold Label 5751 = Like the GE and RCA had a child with the best of both genes. Clean, clear detail with good linearity. Nothing extra special (surprised given reviews I've read), but no shortcomings either. Not as warm as the RCA black plate, but better linearity
RFT/VEB Rohrenwerk Neuhaus factory = bright, tight, detailed sound with sparkling treble, forward mids and less bass than others.
Mullard (Blackburn) & Brimar British tubes = what everyone else says, warm lush midrange, good bass but not distinctive and rolled off treble.
Telefunken ribbed plate and smooth plate = great tubes, linear up and down the frequency range. Detail, and great soundstage, almost like a loudness button turned on from 20 Hz to 20 kHz. That's a compliment, not a dig.
Raytheon black plate = Wow, what a great tube. Superb soundstage, warmth and detail with great bass, strong but not forward midrange and clean, clear treble. Like a superhero who can do it all. Second favorite of the bunch, #1 below.
Amperex Bugle Boy = Double Wow! Something very, very special about this Holloand factory tube in my amp. Matched the Mullard for warmth in the mids, strong structured bass response and a true sparkle on the treble. Much wider and higher soundstage, superb detail, like a fairy sprinkled Pixi dust in the air. My favorite signal tube.
The other thing I've found interesting is the overall sound signature and listening experience is enhanced when matching the signal tube with the gain stage tubes for synergy. I have several NOS brands of 6SN7s and pairing them with a good match signal tube makes a big difference. The match is also dependent on music genre and which speakers are running.
A brighter, more detailed, forward signal tube like the Amperex or Raytheon pair well with the warmer, detailed Sylvania house sound. The more relaxed Mullard or RCA goes great with stronger more linear and forward Raytheon gain stage. The best combination for me is either the Bugle Boy or Telefunken paired with Tung Sol mil spec gain stage. It gives me everything, great fast, deep bass, very warm yet clean forward midrange with clear, sparkling treble and both deep and wide soundstage. I'd give the edge to Amperex + Tung Sol by a pin.
Now, if I was stranded on a desert island with only one match, I'd probably go with the RCA 12AT7 black plate with a pair of Tung Sol JAN drivers. A smooth, linear non-fatiguing sound signature I could listen to 24/7.