According to the Russian standards (GOST) the letters after the tube type signify the following:
V (Russian "B") - increased anti-vibration characteristics/mechanical sturdiness.
E (Russian "E") - increased longevity (guaranteed for over 5000 work hours, unless otherwise stated)
D (Russian "Д") - extra longevity (guaranteed for over 10000 work hours)
I (Russian "И") - optimized for high-frequency/impulse operations
R (Russian "P") - hardened to work in increased radioactive environments.
Each tube can carry up to two suffixes, hence 6P14P-EV (6П14П-ЕВ), is a Russian EL-84 optimized to withstand increased mechanical (vibration) and electrical loads or to work 5000 hours or more under standard mechanical and/or electrical loads.
In terms of using 6N3P tubes I would strongly suggest using Russian ones instead of Chinese. One often overlooked feature of the 6N3P is that it's internal design is absolutely symmetrical (two triodes inside the bulb are mirror images of each other). Because of that, both triodes are close to each other in terms of their electrical parameters, and therefore, are easier to match.
Cheers, Paul