mulveling, thank you, I think you are right and I will follow your advice.
Again, I currently have Mullard narrow long plates from 1959 in the line preamp, early 50s RCA black plates in the driver slot and Mullard XF2 power tubes. This combination works perfectly with my speakers and for my taste, as far as I can tell. The sound is on the warmer side but not at all excessively warm. Resolution, details, dynamics and bass all good, not to mention soundstage, timbre etc. Early 50s RCA black plates in the phono should complement it very well.