I know plus the useless distortion channel. Maybe the worst i ever heard. but the foot swithable extra eq is a nice feature. Also for some reason the studio bass was a 8 ohmn versus the 4 ohm on the guitar version. I will not get technical on why or how or if it makes a difference because i don't know....
If they used the same OT, which I'm sure they did, Fender probably just left the 4 ohm tap unconnected in the Studio bass to discourage trying to use it in a stack. BTW, the Super Twin's distortion channel is only
slightly worse than the Ampeg V-9's, which sounds like a REALLY weak fuzz pedal, but gets helped out by having better overall EQ. The V9's distortion is pointless anyway. By the time you turn it up enough to get any volume out of it, the "clean" channel has more grunt, grind, AND volume.