When I had to integrate a subwoofer to a high quality desktop audio system, I had already worn out the passive alternatives. I knew it was time to get serious, so I purchased a gently used Marchand XM66 electronic crossover. I have used this with great success for the past 10 years.
It has a variable (user selectable) crossover point, and both the high pass and low pass outputs going through a 24 dB/octave circuit. It sounds utterly clean, transparent, and unaltered as compared to no crossover.
It’s a steal. Of course, you could spend 4-5X the $$ on an electronic crossover by one of the audiophile names, but I"m not sure how much better it would be than the XM66.
https://www.marchandelec.com/xm66-electronic-crossover.html