The EL84M tube is made by Sovtek. Most tube supplier's sell the Sovtek line of tubes. I am sure the reason Kora is saying to use the EL84M is because of the voltage used on the screen. The EL84M is capable of the elevated voltages that the other E4's can't/won't live through. I am using the Manley Mahi mono blocks, and they to run the B+ voltages way out there, almost 500 volts. I have tried the Electra-Harmonics EL84EH tubes. They say they also will handle the higher voltages. I have been using them for the last month without failure. I tried the JJ's and they went away within a couple of days. I tried the Sovtek standard EL84 and they to lasted just a couple of days. You can buy some of the older NOS tubes made in Russia with the number: 6Pi14Pi-EB, I have tried those, they work just fine but they sound kind of sterile. I was told by EveAnna (owner of Manley) that the EI tubes will work, in fact she sent me a set of 8 that I have not tried yet. Anyway, I think you would like the EL84EH's, unless the screen voltage is higher than 430 volts, which is what I have trimmed the Manley down to they were 500 volts, in your amp they should work fine, in fact in EH's press release they say "under nay voltage condition". That being said, you can send them back if they should not take the "HEAT". They are warranted for 90 days, and they will go up in smoke in that length of time if the voltages are to high, or they will last thousands of hours. Good luck. Hear is a link to the dealer that has all of the different EL84 tubes,
http://www.newquestsound.com/Default.aspx
good luck..............Bob
http://www.newquestsound.com/Default.aspx
good luck..............Bob