You first want to adjust your mains for the best sound..... including any minor adjustments withe an EQ if you wish.
Then and only then, do you want to add a sub into the mix and adjust that.... for both HZ level and volume.
BTW, when you adjust the HZ you are just adjusting the cross over point of the sub.
To increase the volume at a given HZ, you just increase the sub's volume level.
So, both adjustments work in concert to give you the best setting.
Also, just adjust ether one at a time and do some critical listening before you adjust the other.
You have to play with both adjustment to get the best balance.
60 HZ is a good stating point.
Keep in mind that few LPs have bass that low.... or very little at best.
So many times you sub will be more or less silent used with quality.
You will just hear it when called upon.