OP I still have a loudness contour, bass, treble and balance. I wouldn't give it up for anyone.
Second the actual problem is the sensitivity of your main speakers vs your subs. 10 to 1 says your mains or subs are a mismatch.
Kind of like conventional subs with horns. You have to match speaker types.
My subs are all above 90% sensitivity even GRs OB servos are higher. I use a HO44 10" because of the sensitivity, speed and depth it can reach with a passive and stay very accurate with super low distortion and lower watts for a normal sub.
That drove my wife nuts 25 years ago. She hated it. She likes listening to reel to reel mix tapes. It could have Mic Jagger or Carman Miranda on one of her tapes.
With Macs it's never been a problem controlling subs or bass. Their MX series have sub, bass, treble and variable loudness. The cool thing is from the remote.
We used a C20 for years too, NO REMOTE at all. BUT it's still the best sounding preamp on earth. Samra C20..