It’s not just how low, it’s also how equal in level all frequencies are top to bottom, IN YOUR ROOM.
I have test lp's and cds, this, my favorite test cd has 29 specific 1/3 octave tones.
then you get an inexpensive Sound Pressure Meter (with a tripod screw fitting)
mount on tripod set at seated ear height at your listening position.
make a chart, (I made enlarged copy of the booklet’s track listing)
pick 1,000 hz for example, and adjust volume to your 'normal loud'
now select specific tracks and write down what you are getting.
it can help you refine speaker location and toe-in, see the measured results.
In my case, I have L-Pads to adjust 'Presence' the mid horn output to the woofer output, and L-Pad to adjust 'Brilliance' the tweeter horn to the mid horn. Both sides. Takes quite a while to get it right, but when you do, oh baby!
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If you cannot find that test cd at a decent price, send me a private message.
I did this by ear for so many years, the inexpensive SPL meter and test tones are a treat.