The KT88s take away a bit of the rich midrange and roundness of the EL34s, but they make the amp more aggressive, tame the highs a bit, tighten and deepen the bottom end, and scoop the mids a bit. It still retains its more organic, "wet" tone. I also strangely prefer the cleans now.
You have to add a resistor to get it to bias properly, but honestly the KT88s make it a very different animal. Not necessarily better, but different. I'm very glad that I have the bias switch because I could see myself missing the EL34s eventually.