I post the 2 descriptions:
E34L: This tube has about 20% more head room than the standard EL34 and it also has a deeper, tighter low end. The E34L is also a bit punchier and more aggressive than the standard EL34 which makes it a Great tube for Matchless, Bogner, Carvin, Peavey and all Marshall amps. I have them in one of my old SUNN heads. Incredible sound!
KT77: The JJ KT77 has a slightly deeper low end than the JJ E34L and they have a nice sizzle on the top end without getting brittle and the mid range harmonic structure is very complex. The clean tone is very fat and full and the when driven to saturation the crunch has more of a chunk to it.
And 2 descriptions they write me in answer at one mail:
The JJ E34L's provide a deeper low end and are more aggressive and punchy than an EL34. The E34L is tighter with slightly more headroom and power with a more aggressive crunch, a very articulate British tone.
The JJ KT77's have even a bit more low end extension than the JJ E34L's and they have a nice sizzle in the top end that is not brittle. The clean tone is very fat and full and the crunch at loud volumes has more of a chunk to it and it's very punchy but not quite as aggressive as the E34L, a great tube for rhythm and solo work if you like the pushed mid and upper mid tone that all EL34 type tubes have.