Looking forward to your findings! If you can, it would be great to hear a before&after recording.
My first vintage Starfire, from early 1966, had a single-magnet Bisonic in it and it sounded fantastic. Naturally there are lots of variables at play, including the wood of the instrument itself, the hardware, the strings, etc. In any case, I actually ended up ordering a custom Dark Star pickup from Fred Hammon with a single magnet to try and get a pickup that was closer to the sound of that early '66 bass's pickup.
I didn't have enough patience (nor the equipment) to properly A/B that pickup with the original and actually installed it in a Fender Mustang bass in the middle position... so it's hard to say just how similar the pickup sounded to that old one, but either way, it sounded great! If I recall correctly, that pickup ended up in my friend's '68 Starfire bass, where it still rumbles up a storm to this day.
You may already know this, but at some point in the late 60's, the stock Hagstrom Bisonic pickups found in Starfires started getting 2 magnets, somewhere along the production line... There was some discussion/debate/maybe-speculation that the buzz around adding a magnet (thanks to our Frisco friends) resulted in this change OR that it may have just been how they started coming from Hagstrom OR that it was preferable to have 2 magnets for higher aperture in the bridge position and a single in the neck position, but as Guild used up their stock of Bisonics, they ran out of single-magnet specimens, so the last run of dual-Bisonic Guilds got two double-magnet pickups... Hard to say, but fun to think and chat about!
I should also add, I honestly couldn't tell you which iteration I prefer these days. If the opportunity arises and my friend and his Starfire come down from CT or I go up there with mine, we'll try to do some half-way useful A/B'ing. That said, my 1970 Starfire, as it currently exists, is the best sounding one I've had the pleasure of playing... after tons of swapping and testing, I've found that a standard Dark Star in the neck position and a standard Novak BS-DS in the bridge position is more or less what I would consider the perfect balance. The Dark Star of course is wound a little lighter for a touch of extra articulation and the Novak is wound to vintage Hagstrom spec, so it correspondingly has a bit of a beefier sound. Both have 2 magnets underneath. The last thing I wouldn't mind trying some day is swapping the neck position Dark Star out for a single-magnet specimen, but I love the current combination and tone so much, that I'm honestly not sure it would be worth the effort.