Try Bluebond guitars on 2nd st in center city. Sometimes they have some cool stuff including some used Bluebond basses. Also Chris DiPintos shop in Fishtown. Check out their websites. Guitar show also in Oaks OA July 15-26.
Bluebond has moved, and is now on the 1500 block of Sansom St. in Center City. The owner (Rich) has gotten into buying up stuff from the 1980s, and he has some cool, quirky basses. The back of the shop is now a performance space, where the adult band camp bands rehearse and perform. I've been doing the camp for a year, and it's been a blast.
DiPinto Guitars is great, and I need to get over there. It's been way too long. Chris is really cool, and I miss the DiPinto Belvedere bass I used to have.
I've heard good things about Russo's on Spring Garden street, but I've never been there so I couldn't tell you firsthand.
I've been there a few times, and it's a beautiful shop, with nice displays and instrument testing rooms. They're heavily into Fender, but carry other brands including Reverend, Gibson, and Spector. The selection is pretty good, and includes Fenders you won't typically see at GC and Sam Ash. They have some used instruments, but it's mostly new.
Since 8th Street Music bailed out of Chinatown over 10 years ago, we hadn't had a great shop for new instruments in downtown Philly, so it's been great to have Russo's open such a cool space.
There is also Bridgeset Sound on South St., which carries G&L and Yamaha, but it's much smaller. When they first opened (2014?) they were a G&L dealer, then they largely stopped selling new electric guitars and basses and dropped G&L, but they're back on and carry both USA and Tribute models. Bass selection is tiny though.