Parts Express carries the Harrison Labs fixed inline attenuators in 3, 6 and 12 dB versions. Right now, your guess is as good as mine which you might require but you only need them for the amp with higher gain.
My experience would advise to stay away from variable, potentiometer based units.
Passive volume control attenuators, such as Goldpoint, can be too rough for fine tuning biamping configurations and have fixed input impedance. Which brings up the other consideration of parallel amp impedance getting reduced and whether the preamp can handle that. Usually not a problem for solid state preamps.
Fair warning: Many biamping experiments, passive or active, end in failure because it's far more complicated than most people anticipate or are prepared to understand.
My experience would advise to stay away from variable, potentiometer based units.
Passive volume control attenuators, such as Goldpoint, can be too rough for fine tuning biamping configurations and have fixed input impedance. Which brings up the other consideration of parallel amp impedance getting reduced and whether the preamp can handle that. Usually not a problem for solid state preamps.
Fair warning: Many biamping experiments, passive or active, end in failure because it's far more complicated than most people anticipate or are prepared to understand.