Maybe you should have increased the level of the subwoofer when you lowered the low pass frequency (that is what REL usally recommends, and I tend to agree). But it is just a thought. If you really want to know/see, get a calibrated measurement microphone like the UMIK-1 and do a measurement in REW (but not for the fainthearted in computerland). Your original 75 Hz was certainly too high without high passing (i.e. cutting the low frequencies) of the main speakers.
As for the Antimode. If you do not have a preamplifier output, you can connect the sub at speaker level and then use an attenuation cable (that is what I do). Look carefully at the connection examples at the DSpeaker website. The cheaper Antimode 8033 Cinema model is probably all you need.
As for the Antimode. If you do not have a preamplifier output, you can connect the sub at speaker level and then use an attenuation cable (that is what I do). Look carefully at the connection examples at the DSpeaker website. The cheaper Antimode 8033 Cinema model is probably all you need.