Ideally you would want to insert a high pass filter AKA electronic crossover, between your preamp and amplifiers. If your amp is an AVR this feature I believe is built into most of them. If you are not using an AVR you have a few options at varying price points. A lot of people seem to like the miniDSP, DBX makes an economical analog crossover and there are other options, including some that can sell for several thousands of dollars.
I’m not sure that there is an ideal crossover point, kinda depends on the style of music you like, how loud you like to play, the kind of subs you have and to some extent your room acoustics, If your subs are close to your mains you could crossover as high as 200 Hz if your subs can cycle that high. If your subs are not inline with your mains you would want to crossover no higher than 80 Hz. I play a lot of bass centric music at rather loud volumes so I cross my front subs at 173 Hz and my rear subs at 78 Hz.