The Gretsch Junior Jet (G2220) pickups are single coils. Apparently the original Gretsch single pickup model was a humbucker, but when the two pickup model came out and fender claimed they were humbuckers, there was a real hue and cry from people who bought them and discovered they were single coils. They actually were compensating customers who bought them and then discovered they'd been misled. I liked the fact that they're single coils and I've had a lot of people I played with in bands comment on how clean and articulate they sound.
After I'd gotten used to the Gretsch single coil sound, I took my Hofner to practice one day for the first time in a long time, and was really dismayed with how muffled and boomy the bass sounds......I don't know if it's just the pickups or if the change to the D'Addario strings is a factor, but I'm quite dissatisfied with the sound now. That's what's got me thinking about changing the pickups. I've had a couple of suggestions repeatedly...One being to change components in the Hofner control panel, and the other being changing to the Firebird pickups. I haven't heard the Firebird pickups, so it would be a risk to just blindly change them without knowing what to expect. I do know that something needs to happen - As it stands, the Hofner's destined to be a stand queen that never gets played.
I looked on eBay for the Epiphone pickups you mentioned, Guerilla Electro, but can't find them. Any of the Firebird pickups shown are described as humbucking, and when I search for Epiphone single coils, the only search results are for P90's. I did like the idea of something that's a direct bolt-in....I can see where trying to change the style of pickups in a violin bass might turn into a can of worms.