Soundstage, layering and tube differences

,are more 2D? I just replaced my 10 year old SED’s with the Mullards ( which still need burn-in), but they are not nearly as 3D in my amps and are reported to be such.

Yeah there is alot of hype around power tubes.
I can tell you one thing, fair,
the 6550 Svetlana is perhaps the best all around power tube ever made.
Very close is obviously the twin brother to the 6550 , the KT88. , KT90’s are close, thats it,, I do not think 120’s, 150’s match the previous 3 in midrange acoustics.
EL34, kt66,77 can not compete with the 6550/kt88.
russia makes the most reliable 6550/88’s.

Do some tubes resolve the actual recorded room acoustics better or is it a matter of harmonic distortion giving the illusion of soundstaging and 3 dimensionality??

Yes. They are lower distortion, sometimes due to geometry being a bit lower distortion on account of how the electrons flow within the tube, the amount of actual vacuum, how consistent the cathode coating is, stuff like that.

If you like how well EL34s do, triodes can do it even better as they are more linear.

Try a pair of the new version Gold Lion KT77s...a great sounding tube.

You know that you need to burn in your tubes and make an evaluation ever 50 hours.  Around 200-400 hours you can then see what you system will sound like.  Take some time and evaluate your results..

Tubes need zero burn-in...zero.