Hi Howard,
Well, here's a really basic tube desciption..
V1 is technically the input tube on your RM4 or RM amp..That is always the tube closest to the input jack...This one has a major impact on all modules as it's the the first tube hit by your signal...It's also important to note that the Phase Inverter tube (closest to the power tubes) is also important...Having one that is balanced/matched is recommended but not always necessary - a side note is that a lot of people here prefer a 5751 or lower gain tube in the PI as it cleans things up a bit...The other preamp tube in your amp is the loop buffer and just needs to be a tube that works...Not a spot that has a big impact on tone...
Now to the modules...
If you are looking from the front on the module, the tube on your left is what I call "module V1" since techinically the true V1 is in the amp...Anyway, module V1 is gonna be a very important tonal tube and will have a big impact on the way the mod responds and feels...You should try a few different tubes here as they will all have some differences..."Module V2" or the tube on your right when facing the module has more to do with the perceived gain of the module...It still impacts tone, just not as much as Mod V1...This is where I like to use a High Gain tube in many cases...
As far as brands...I won't even go there yet...This is where you will get a lot of variety in your answers...Some here believe that a single brand of tube should be used in all spots for consistency as well as simplicity...Then there is another group who prefer a couple different brands for various spots - i.e. a TungSol in V1, EH in Mod V1 and a Ruby HG in mod v2....Then there are some who experiment forever with changing tubes until audio nirvana has been achieved...Then a week later, the changing starts all over again...
Generally, lower gain tubes like 12AT7, 12AU7, 5751, etc will give you a cleaner tone...Great for low gain mods and some like them in high gain mods to tone down excessive gain and add clarity...Also liked by many as the Phase Inverter too...Standard tubes are obvious...And high gain rated tubes will have a hot output and can add to perceived gain...
Hope that gets you started!
Cheers,
Mattfig