If your active, and I assume powered, subs have their own cross-over and gain control, why not simply connect them to your amp via the sub’s high-level speaker terminals (if they have them), not your pre? (share the binding posts with your mains). If powered subs, this is very safe for the amp as you are simply sending the shared signal to the subs, not powering them. In this way you can simply adjust the subs to the mains in terms of integration.
I do this, and my subs are in ’stereo’, albeit, no, I do not expect to hear them in stereo, it’s just easier that way in my set-up. They only activate when necessary per my crossover and gain setting, basically supplementing the mains.
Now, if you want to control the subs gain for each selection of music, perhaps your idea works better, or, perhaps you have no high-level terminals on your subs. For me, it’s adjust/integrate them and done.