Every time I've added a sub, or subs to my system, I've had all sorts of challenges getting them to integrate properly.
My last struggle was integrating a pair of JL subs in a new, larger "man cave", where the room had all sorts of obnoxious resonances.
Using a sound pressure meter and a CD with sweeps I was able to improve things (helped me place bass traps in the room), but the system still didn't sound right.
Then I discovered a program to download, "REW" (Room EQ Wizard) and a " miniDSP UMIK-1 USB Measurement Calibrated Microphone" (thank you Amazon), and with those tools was able to get the crossover point, crossover amount and volume adjusted, and now the system sounds awesome.
You would need a PC (not sure if REW works on a Mac, bet it does, it's a Java app), and the microphone plugs into the standard USB port on the computer.