C for Chinese and R for Russian, is an internal naming scheme by GrooveTubes. Their description of the tonal characteristics of each seems contrary to commonly held beliefs, at the risk of generalisation, Chinese tubes are preferred for grainy Marshall type tones:
but in GrooveTubes' case the Chinese ones are labelled as smooth and round...in my book smooth and round means low dynamics, which is generally attributed to Russian tubes...ymmv.
How were they originally installed V1+V2 Russian the rest Chinese? Or V1-2-3 Chinese?? The five tubes all perform radically different functions so they may be selected for their particular qualtities (linear/compressed, raspy/grainy/soggy/mushy, high gain, etc.) The input tube for example, may be set to unity gain and is only there as a buffer for the input. (maintaining the tonal character of your guitar)
Logic dictates that V3 and V4 should be linear and clean (FX recov, master volume and reverb) but using a heavily coloured tube in those locations could really define the amp or take the edge off digital effects.
With the "bad" tube in the reverb location (V4) you should be able to hear that as you crank the reverb. It also means you could "park" the bad tube there without much notice.
I still think that perhaps only one tube is failing, so spending 5*20$ is a bit silly. You could buy two radically different tubes (but both 12ax7s) and switch them around in the V1, V2 and V3 locations to fine tune your tone. The spares will keep forever.