I currently use 9 and I have used 8 gauge strings (with proper setup after changing gauges) with no tonal difference at all. I've seen many A/B videos online, A/B recorded my own guitars after changing gauges and there is really very little to any truth about tone and string gauge. Now, if you attack the strings differently, maybe you play softer on light gauge strings for example, then you'll have tone changes. But nobody can say ZZ Top's tone is thin and he uses 7 gauge.