I own modified AC15 with Alnico Blue speaker. I found the EL84M simply great. Only they need to be biased right. Compare to most of others EL84s I should to boost the bias much hotter. With a cold bias the sound is sterile and dull. I got the original datasheet of Russian 6P14P-EB ( EL84M in TAD ) and the min. bias for class A is 40mA - typicaly 48mA !!! I have now 43mA.The Tubes are singin and sparkling - very lively sound. No hum, no harsh, no hiss.. I tried many others EL84s ( GT, JJ, Mesa Boogie, Engl ) and EL84M are the best.
It's a bless when you have an small combo that goes microphonic with any other EL84 but, sonically, has a very sterile tone.
These matching valves where the bizz, I bought five valves in all these for the power amp gives it a sweet sound thank you TAD for prompt service and delivery.
I will recommend you to friends and fellow musicians.
Paul Chuter, UK August 2008: