Wow, I tried Roto 77 Jazz flats, short scale, on my 1969 EB-0, terrible! Too loose, and stiff construction, so that the E especially had terrible performance, bad intonation and poor note stability, very clunky. Also a really dark, all-bass tone.
Original strings on the Gibsons were surely flat wound.
I haven’t yet found the perfect string! I’m trying to get a plunky flats sound like in Jethro Tull “Living in the Past.” Also a very blurry, bassy tone like “Come Together” and an overdrive tone like Jack Bruce in Cream, but those two are easier to get with this thing. The Cream and Tull were recorded with an EB-3 apparently.
Some other strings I’ve tried:
Dunlop Flats 40-100. Very high quality and great intonation but a little too loose, and bassy. Sort of a modern flats sound, bassy but with some brightness on top.
GHS precision flats. 45-95. Very nice, old school with some growl and warmth, good performance, but also rather loose and bassy.
La Bella LTF 42-100. Really lovely strings, sounds reminiscent of a double bass, but again quite loose and bassy.
Anyways, my thought initially was to go with those more light gauge flats to get good performance, not too clunky/bad intonation E which can happen with stiff flats on a short scale. Despite the fact that short scale needs heavier gauge to get the same tension as with long scale. But those light flats were just a bit too loose and sloppy feeling and the sound is too bassy, not enough mid range “plunk”. The wide excursion of the light gauge strings produces a lot of bass I think, also lots of bass is inherent in the EB-0. (The LTF work great on a Guild Starfire though, which has a slightly longer scale (30.75 vs 30.5”) and single coil pickups, and takes medium scale strings due to the longer bridge tail). I have the treble kill capacitor unhooked from inside the EB-0, and turn the bass on the amp way down, so that helps.
Finally tried the GHS brite flats 49-108 on the EB-0, as often recommended on this site. Actually is a ground round. Flexible, good performance, but also high tension so you get more chunky mids and defined attack. I don’t think it’s perfect, though, maybe too much tension, especially a little worrying on the old bridge and neck. And seems a bit grabby though I cleaned with alcohol and gave them a coat of hand lotion. Pretty great though, just a nice generic sound to the string, and not much round wound growl.
Sorry about the long verbiage with no great conclusion, but hopefully some good info.