What coffee do you use?
These days, I don't opt for coffees that are "sour". Or bitter (much though I like those notes in other facets of my food or cuisine).
I have come to realise that I prefer smoother, sweeter, brighter, "cleaner", clear tasting notes: In practice, that tends to mean African coffees, but - and it is a big but in the world of espresso - these tend to be relatively lightly roasted.
This is Four Barrel's "Brass Tacks" blend. Which is described as "crisp and courageous". When I brewed the same beans in my Aeropress before I got my espresso machine, it was bright, not sour. I would say this is on the lighter side of roasts.
Maybe I'll try a darker roast for espresso. I generally don't like the brighter tasting notes. I prefer earthy, clean, chocolatey notes.