6550s and KT88s are normally interchangeable, and on some amps such as the Cary V12i, all three you mention are interchangeable. So depending on you choice of amplifier, you may be able to run any of the three.
The most melodic of the three that I have heard was the EL34 run in triode mode (in a V12i) in a push-pull configuration. The most dynamic and detailed I have heard is a 6550 run wired in tetrode mode in a single ended amplifier.
I make reference to the amplifier configurations as I believe that has probably a greater influence on the sound than the actual vacuum tube you choose. Besides, there is a world of difference in sound between a cheap production and good NOS example of the same tube.
In your price range youd probably do better looking at the used market for either EL34 or 6550/KT88 push-pull amplifier to drive your speakers. There are a number of Chinese made amplifiers (ASL/Jolida) that are very competent.