You weren't clear if the tubes you installed were the the same type as the factory installed. If you replaced the 12AU7 and 6922 with the same types it should NOT void the warranty in my opinion. In the close to 40 years I've been an audio electronics hobbyist (building, repairing and modding - I've got the scopes, distortion analyzers, meters, freq generators, etc.) I've never heard anyone else state that changing a brand of tube voids a warranty. I'd strongly question that. I'd want to see a copy of the "European ISO" regulation with my own eyes.
Now if you were experimenting with different types of tubes there could be an issue - the pin-out or operating characteristics could be different and cause a problem.
However, step 1 is to figure out what is currently wrong with the amp. If putting the original tubes back fixes the problem, you may have had nothing more than a defective tube. If something more is wrong, it still needs to be found. No manufacturer has 100% quality control and it could be that while a part passed the initial QC tests it might have been marginal and was doomed to fail in a short period of time no matter what you did or didn't.
Now if you were experimenting with different types of tubes there could be an issue - the pin-out or operating characteristics could be different and cause a problem.
However, step 1 is to figure out what is currently wrong with the amp. If putting the original tubes back fixes the problem, you may have had nothing more than a defective tube. If something more is wrong, it still needs to be found. No manufacturer has 100% quality control and it could be that while a part passed the initial QC tests it might have been marginal and was doomed to fail in a short period of time no matter what you did or didn't.