Are there passive attenuators that don't zero volume and only offer one input ?You've just described the Lightspeed Attenuator to a tee.
Sure, Khozmo and Hattor. I just recently received a Khozmo dual mono passive attenuator with separate left/right channel volume adjustment, one input, two outputs (one for subs), all balanced XLR ins-outs, display, and remote control of all functions. I use it for remote volume adjustment in conjunction with a unity gain buffer. Discrete resistors with Takman REY metal film shunt resistors and a Vishay "nude" Z-foil series resistor.