From one old guy to another... The Rel G-1 has a volume control for both volume and crossover. Mine is great but mostly set and forget. Expensive but world class. Good luck.
I think there are others but I use a Velodyne SMS=1 with my sub. Not only does it EQ the sub for the room but it works great if you want to experiment with crossover slopes. All via remote including remote volume, phase, polarity, slope.etc You can find them used for $250 =$300
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