There were different tube manufacture sites for the production of the 6N1P as well as different versions and periods of time when the sound produced was different due to varying manufacture quality control standards. I had a friend who was an engineer that worked with tubes in the USSR. He explained a number of years ago to me, the different 6N1P types and the reason for the different sounds as well as differing quality within the same designation. Just like tubes from the US, there was a golden age and one that was devoted more on just getting the tube out the door. Since the 6N1P as a tube was used longer in circuits, later in the 20th century, than tubes in the US, because of military use, they are often still of higher quality than the corresponding time period of tube production in the US and other areas. Tubes were still being produced in the USSR long after almost all tube production had ceased in most other countries.
So there will be variables and obviously, with this, sound differences in the 6N1P, not only with EV or IV or E designations but on production facility and when they were produced. A general sound quality can be described for the 6N1P but not in absolute terms. There are 6N1P's with better bass performance and some that are dry and a little gritty in the upper frequencies. I find the best to be from the mid sixties in black flat plate but they are nearly impossible to find now. To me the tube just mentioned a 6N1Pe, is the most balanced and smoothest sounding but it also has a slightly different design because of application, which was in pure analog circuits.
I'm convinced you need a larger tube collection. 
