Night and day, in my experience. I bought a used Cronus based on spectacular reviews. When I first tried mine, with the KT77 tubes, I heard bloated, unfocused bass, average mids and just OK the rest.
I decided to try it with EL34's - often spoken of as sweet tubes, and the difference was astounding. Bass tightened, focus sharpened, highs sweet, clear. Everything perfect. I chose Electroharmonix EL34EH for about 8 bucks a piece.
My dealer persuaded me to try Groove Tubes EL 34 as being better and I found them OK, but a bit unfocused and they were 130.
I replaced the small tubes with Telefunken and Amperex Bugle Boy and the difference was better, but not as HUGE as the power tube difference.
Should you have other questions, feel free to contact me.