It is true that the EL84M is more robust, due to the fact it can handle high anode voltages better than the regular EL84 can. Kora used transformers that are designed for 115V primaries when we commonly have 126V available today. This means that for nominal 400V rails, the Koras actually have 440V rails, which causes the tubes to over-dissipate.
When I repair Koras I lower the rail voltages, and I reduce the Q point a bit, so that any EL84 can be used (even though the "M" remains better). This eliminates this issue. Most of my audio buddies have Koras that I rebuilt many years ago and they have had zero issues and rarely need new tubes. And they sound incredible.
As for putting Svetlana tubes in, this is not the problem. The reason an issue came up is because you must adjust the bias when you retube these amps because they have a fixed bias - they are not autobias (cathode biased) like most other amps on the market today. You can get much better performance with fixed bias, but it requires knowledge and skill to replace the tubes.
