That makes sense if you’re using the speaker level input. There might be other options, though you’d need to be careful and run the numbers.
With the speakers being so efficient, depending on your listening levels, the speaker level voltage could be within the limits of the line level inputs of the sub. There might be a way to modify the sub’s high level inputs for less attenuation as well.
You could run a sub out from your preamp at line level, but attenuate the signal going into the power amp and to the speakers. This depends on your amp input impedance. A passive pre amp would give additional adjustment. I think some attenuators can have roll off if the output impedance of the pre is too low.
I’m sure there are variations of these, but it would depend on your specific setup and equipment.