This game is really good! You've made me try until I beat it! And then try again!
Few things to notice:
I had muted the music, and when I got to 1000 it unmuted the music automaticaly although the icon was showing as if on mute;
I never thought of using or used the back button;
The NEXT TILES section could still work after reaching 1000;
It seems HIGHSCORE CLASSIC doesn't update on the go, as it was still showing 800 when I reached 4000;
Maybe you could let us combine x2 with x5 until 1000 and then combine anything?
Very solid game though! Well done!