I was already totally hooked on guitar when I saved up and got my SG and LP (separate purchases). I was quite content with the amps I had at the time. Well, until I "discovered" Mesa amps. They sounded so cool and neat that I ordered a Son of Boogie when they first came out. My plumb line for amps changed at that point. I willingly drank the Kool-Aid.
Having said that, the real "game changer" in my playing came about when I got my hands on a used Mark IV. Fantastic amp! The thing that made this one a game changer was not so much its tone (I was already a fan of the Mark series) or it's being a Mesa (I was already a fan of theirs). Strangely enough, it was the Mark IV's unforgiving character. It was such a step above everything else I had ever played through in that it faithfully amplified every thing I did - especially the mistakes and my sloppy playing.
The Mark IV made me concentrate on technique more than I ever had before. In short, it forced me not just to learn a song well enough to check it off of the list, but to learn the song and make it sound like I really knew the song. My playing moved up a notch (please note that I do not claim to be a great player, "hopefully adequate" is the term I use).
For me, that was a huge game changer!