Is your sub self-powered? If not, I would advise selling it and getting a self-powered sub with built-in equalizer.
Sub receives l and r from pre-amp, takes the low bass, sends only upper bass, mids, highs out to amp. Importantly it's volume is controlled by the preamp.
Adjust Sub's crossover point; adjust Sub's volume control to match your main's at your most frequent listening level. Now any volume change at preamp goes to sub and amp(s) cohesively.
Sub receives l and r from pre-amp, takes the low bass, sends only upper bass, mids, highs out to amp. Importantly it's volume is controlled by the preamp.
Adjust Sub's crossover point; adjust Sub's volume control to match your main's at your most frequent listening level. Now any volume change at preamp goes to sub and amp(s) cohesively.