About 9 months ago, I pulled 4 of the STR 420's out of my 2:90 and put them in my 60/100 IIC+ (ie, not simulclass). For the previous 6 months, I had been using 4 new STR 440's direct from Mesa. I didn't really like the 440s. I liked the 420s a lot better. They were a bit softer-edged than the 440s...in a good way. Of course, the 420s were a lot more used than the 440s, so maybe that had more to do with it.
About 6 months ago, I got a quad of Winged C. I like these the best so far.
All of the changes in sound have been subtle...440 -> 420 -> Winged C. I'm not claiming that I'd hear a difference in a double-blind placebo trial or anything. But, I do feel that there was a difference. Unless your old tubes are beat to hell, don't expect a jaw-doppingly huge change when you put in new ones.
My opinion.
Chip