I have used Manley Reference 200/100's for the last few years with either Maggie MG3A's or Stax F-81's; both wonderfull combinations in most respects. Luckily, I don't listen at particularly loud levels so this dilemna is tolerable: In triode mode both these power hungry speakers benefit from a much more refined presentation as far as timbre and microdynamics. Instruments seem a bit leaner, but with their harmonic structures much better preserved, and their leading edges more incisive in a natural way.The soundstage appears to be somewhat smaller, but the dimensions of it are far more obvious, and there is a lot less texture in the spaces between instruments. Microdynamics are actually more natural, although ultimate dynamic range is compressed. On those occasions when I feel like listening at louder than usual levels or the music is extremely complex and dynamic (big orchestral works and some rock) the music reaches a dynamic ceiling sooner than it should. Since on those times, I'm usually not doing serious listening (working out or running around the house), switching to tetrode is the ticket. I can play the system at louder levels without obvious compression, and although I find that the sound, in general, gets a bit thicker than life with less harmonic detail, the increase in bass drive makes up for it. I am always reminded of when I switch between my Vandenhul MC and my Grado. Overall, I much prefer triode mode.
By the way, from Evana Manley herself: "You can switch between the two modes without powering down, but MAKE SURE THAT YOU MUTE YOUR PREAMP FIRST!"
Hope this helps and Happy New Year.