Which tubes? My amp can except El34, 6L6, KT88, an

Sort of like asking which flavor of ice cream you should choose? They are all good but different. I am a fan of 6550's and EL34's out of the four you mentioned. I would think that you won't get the power from the EL's that you will from the 6550's or if you do they can't last very long. I have Vandersteen's(not 5's)and run a 6550 based amp which is a great match. I would start there and use them as a baseline or benchmark to judge from. You may end up eventually running one of the others, but good Svetlana 6550c's can be had for about 1/2 the price of decent KT88's so if you don't like them you don't have as much into them.You may be able to talk with your dealer or with Quicksilver and tell them some of your preferences and they may be able to make a rome qualified recomendation. I am going by the idea that if you like the same speakers I do you may like the same tubes.What is in the amps now? I'll take coffee ice cream, thanks.

I have a pair of Vandersteen Model Five speakers with a CAD280 V12i tube amplifier with EL34 output tubes. I have it set to the triode mode for 50 watts per channel and with the Model Five built in subs there is enought power. I find the EL34 more detailed and musical in the midrange than the 6550 or KT88. Of course it is a matter of taste. If you purchase current tubes in production and not NOS you can try more than one type of tube without a significant cash outlay.

I am a fan of all four tubes you listed. My current favorite has to be the 6L6 though. Seeing how you'll need a dozen, they will probably be the cheapest of the four. They are not the best at low end, but your Vandy's amps will take care of that. The midrange is to die for. Female vocals are just awesome. Stunningly awesome. All the midrange magic of the EL34, but sweeter (to me the EL34 sounds a lot like a 6L6, only much drier). They produce a great 3D soundstage. They do pretty well with rock, hip hop and techno as well. I've had best results from Russian 6N3C (a variant of the 6L6), they are fantastic. The Svetlana 6L6 is pretty good. You might avoid the JJ Tesla 6L6, as it has a compressed sound. I liked it at first, but quickly tired of it. Too much loss of micro and macro dynamics. As Maxgain alluded to, it's very much a personal choice. Good luck and happy tube-rolling!

ps - I'll take the rocky road in a regular cone