| I made the jump from the 420 to the 750A very soon after it came out and it was (to me) a big move in the right direction. It was a bit louder as you would expect and though the tone itself was relatively similar, the extra muscle under the hood gave the 750 a wider\fatter and more open sounding tone to my ears. It is bigger sounding overall - where the 420 sat nicely in the mix, the 750 sits AUTHORITATIVELY in the mix. The 420 would compress and the tone loosen up (in a negative way) when really pushed and I never experienced this with the 750 - or maybe its just because I never had to push it as hard.
In addition to the amp being more powerful, there are several innovations included with the 750 (taken from the 550), including the "switch". Others more knowledgable than me can comment on that... | 
