A lot of this is up to a persons ears....
A 71A is lower power than 45 and operate on 5 volts for filament rather than 2.5. A 45 is half of a 2A3 tube. 6B4 is same as 2A3 with different base and higher filament voltage.
Push-pull output gives the best sound with least distortion. Some like 2nd harmonic distortion so use single ended output. How do you measure "warmth".
Norm
:Here is an interesting subject.I have heard that 45 power triodes have a sought after tone.What does that really mesn?Is this tone exclusive to 45 's only or do 71A's and 2A3 have basically have the same tone ,and what about 6B4G's etc.Is there a certain warmth to the tone or frequency response to 45's.Also how would a 45 tube sound in a single ended output compared to a pushpull configuration.And what about the other tubes 71's 6B4G etc.
That's what they'll tell you, but any commonly-available triode used at its normal operating point will sound the same. I doubt if you could tell the difference between a 45, 6B4 or 6CK4. But they look different, and octal tubes just aren't exotic enough. "Tone" is a weasel word for "I want my amp to look distinctive".
45s have the advantage of supplying enough power for the average efficient speaker, and being available but not "too" available. 71As don't have enough power. 2A3s, 10s and 50s have plenty of power but are considerably harder to find, since fewer radios used them.