I hope it is not the case, but, I suspect that you will have to be concerned about one tube becoming prematurely weak in your parallel SET setup. I have not heard of any "cure" for this condition, and certainly Audio Note amps were prone to this happening. I have a pair of Kageki parallel SET (2a3) that has this as an issue. The thing is that the amp continues to sound very good even when one tube in a pair tests quite weak.
Good luck on your build and on your search for 3oobs.