It's an 8" driver and it can handle but low frequencies without distress. If you really want to set a low limit you should aim for a 60Hz or less. Do you really want your 15" subs handling the upper bass region?
Limiting a full range driver
My speakers, Reference 3A NEFES, have 2 full range 8" drivers. My question; is it better to limit the frequencies to 100 Hz and higher. My thinking is that the lower, sub 100 Hz signals interfere with the midrange too much with their larger cone movements. Or, is there no benefit to cutting out these frequencies?