How to go back to "All Artists" in iTunes
The only way I know short of scrolling with the mouse is, after having pressed Command-L, you can press the following key sequence to get to All Artists: Tab, Tab, A, ↑, ↑

This assumes you have an artist's name starting with the letter A. If not, then substitute A with the first character of the artist's name that's directly under Compilations in the sidebar.

In other words, after having pressed Command-L, pressing Tab twice sets focus to the now playing artist in the sidebar. If it's not the artist directly under Compilations then pressing the appropriate key to get there plus ↑ twice takes you back to All Artists.