d'addarios are great strings. i use them on my les paul but prefer fenders on my strat.

going from 10's to 9's? most people are going the other way, but depends on the music you play. stick with the 10's if you can, the tone is much better.
I've been imitated so well I've heard people copy my mistakes.
- Jimi Hendrix
it depends on what guitar and setup you have
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Yeah I use 13's on my strat to fatten up the tone. But I use 9's on my PRS since that's the way it was set up by that factory and the tone is amazing no matter what size you use.