Only time I've been was LA & SF back in 2009.
The Guitar Center in Hollywood was a cool shop to check out. There was a couple of other shops nearby at the time, not sure if they're still going though. I know the little Boogie shop across the road closed though. Still kick myself I didn't do my damn homework and check out the Elm St house just down the road while I was walking around the town.
Trutone Music in Santa Monica was another cool guitar shop. Whisky Au Go Go, Rainbow and Roxy were all worth a selfie visit.
Amoeba Music was frikken amazing. I think it's relocated now too, no idea if it's still as huge. I could have spent a whole day in there.