Differences b/n KT-88 and EL-34 Amps?

I would say not to focus on speculation about intrinsic tube sounds. Their particular implementation in a specific and total circuit is more of a determining factor in how any amp actually sounds. Even within a given circuit, it is my opinion that there are other parts, such as the output transformers or coupling capacitors, which contribute at least as much to the overall sound as the tube types, but parts selection is just a fraction of the picture. Generalizations about tube types really won't help you choose which amps to audition, as different sounds are possible from different amps using the same output tubes, and different brands of the same tube type also sound different from one another. But as far as the amp goes, the designer's skill in creating and implementing the whole product is more critical to assess through listening than prejudging the means he used to get there. To put it another way, when was the last time you got interested in a solid-state amp because of the variety of output transistors employed? KT-88's and EL-34's are actually more similar to each other than they are to 300B's, which you could look at as being roughly analogous to the difference between bipolar and mosfet transistors in figurative terms, but still, one has to listen before you can conclude anything about the sound. If it helps to say it, EL-34's will be more economical, and are more commonly used in moderately-priced amps than the potentially more powerful KT-88's or 6550's, but any of these can be used to make good-sounding amps, and there are even some amps which are designed to accept several different types of output tubes if you fancy experimentation.