This very issue -- how to apportion audio frequencies above the desired cut-off point to the mains, and leave the rest to the subs -- defined my "sub journey" for years until I finally put some time & $$ into a solution:
It’s an electronic crossover. Mine is a single-ended, variable crossover point model from Marchand, a designer well known for high quality crossovers. I bought this one used for ~$500, and it has worked flawlessly for years. I run the output of the preamp to the input of the crossover, which splits the frequencies into 2 output pairs:
- High pass: Frequencies above the crossover point I chose, 75 Hz, go via RCA IC to the class D monoblocks (bel canto 600Ms) that drive Harbeth 30.1s in a nearfield/desktop configuration (strange but true)
- Low pass: Frequencies below 75 Hz go vis a 2nd RCA IC to the JL Audio e110 sub, in which I have the internal crossover bypassed
This crossover has trim pots for each channel to adjust the output of the crossover in +/- 1 dB steps. Once that’s set, all volume control is done by the preamp.
The Marchand uses 24 dB/octave crossover slopes, which work quite well in my desktop setup with a variety of speakers. This component is very transparent: I’ve compared the sound with direct wiring through the amps to the speakers (no sub) vs wiring through the crossover (also no sub). I can’t hear a diffrerence.
Before using an electronic crossover, everything I did was stumble, fumble, audiibly inadequate. This crossover instantly made the problem go away.