Velodyne DD18 Owners - Tips and Guidance?

I recently acquired a Velodyne DD18 in perfect condition and I wanted to see if I could get input from fellow owners on how to get the most out of the subwoofer.

Should I take advantage of the low-pass filter? I'm currently doing that, sending just 80hz+ to my speakers (Audio Physic Avanti III) and letting the subwoofer handle the rest. However, I'm wondering if I'm losing any detail or quality by adding that step. Do others use this approach?

Placement: I find I have to place the sub far awayfrom my turntable because at high volumes I seem to get a lound shuddering effect, likely because the sound pressure or vibrations are somehow jiving with some natural frequency in the tt setup. Playing digital there is no problem.

I have the sub setup behind my speakes, about 6 inches from the front wall placed on a Sistrum 101 stand.

I have the volume at about 12 as I don't want it making its presence too obvious. I find the software/alignment quite quirky and not so user friendly. I can't seem to get any sound test frequency to measure the room response, so I just do the standard 1-2-3 alignment holding the mic roughly where my head is in the listening chair.

Anyways, I know the above doesn't include many specific questions but overall am just looking for some best-practices tips on getting the most from this subwoofer. I listen to a broad selection of music and this system is also for movies.