[Boatanchors] General question on substitutions

[Eugene Hertz]

I am investigating a problem in my central electronics 100V transmitter. I *think* the problem is with the first mixer which is a 12BY7. I began looking to get this tube and noticed there are subtitutions available. Other than the 12BY7A which is a "slow start" version, some websites list substitutions as 12BV7, 12DQ7, 12GN7. However not all websites list all of these as subsitutes for each other.

The GE datasheet for the 12BV7 is the only sheet that directly mentions the 12BY7 saying "In application and characteristics it is related to the 12BY7" 

Some sites like NJ7P list a "preferred" substitute as the 12DQ7 and a general substitute (not preferred) as the 12GN7 but does not mention the 12BV7 (again which is mentioned as a "related" tube in the GE datasheet.

So what does it all mean? Am I making much ado about nothing here? Do I just pick whichever I can find in my tube spares and try it? If it works leave well enough alone? Is it possible that one of these is lower noise than the others and could be beneficial? Or any other benefit? (i.e. Oh yes, people stopped using the 12BY7 in favor of the 12xx7 long ago because of its superior properties...)

What sayeth you all tube experts?
Eugene