Most of the combination are good for all type of musics and are ok for mixing many kind of tracks.
In fact i use some tube combination for long (>2h) listening session when i choose a majority of track with same type or when i decide to listen complete opera. I do not switch and warm when i change track
It is just that i like warmer sound with most of my blues,jazz, pop and soul listening but prefer a more analytical listening with opera when lot of voice and large orchestra need to be reproduced with a lot of dynamics.
I only have one ECC32 and a pair of ecc31 remaining. You are completely right ECC32 and ECC31 have the same sound signature i also have fdd20 which is a cousin for my ears but i do not use it anymore due to 12V heating.