I take it you're working with a Herbert?
Seems like a lot of work with regards to re-biasing every time a tube swap occurs. What I can tell you is I've used all of these tubes... I used them in a VH4, VH4S and an Uberschall Rev.Blue and a TJ.
EL34's - classic beautiful rich overdrive and that vintage grunt that just can't be beat. Brilliant and very common - it's the sound that makes ya grow a rubbery one when you hear it loud
6550's - powerful, crisp, great low end, great high end, great articulation throughout. Very potent and loud, and smooth overdrive and no bumps in the sonic spectrum. I have these currently in my VH4S and they're outputting more than 6L6's or EL34's. They're bloody loud and knock you on yer ass.
KT88's - in my Uberschall, after having EL34's in them - KT88's in this application are the bomb... Kind of a melding between 6550's surgical crispness and the warmth and tone of the EL34. Great output, great clarity, and a really beautiful overdrive that's....in one word....RICH.
Hope this helps!
V.